[IMP] mass_mailing: correctly define snippets toolbar template

* Split snippets into templates and use the t-snippet directive
* Correctly handle snippet thumbnails thanks to t-thumbnail and not
with custom css

addons/mass_mailing/static/src/css/mass_mailing_editor_thumb.css

-/* ---- thumbnail ----  */
-
-.oe_snippet.o_mail_block_banner .oe_snippet_thumbnail_img {
-    background-image: url("/mass_mailing/static/src/img/blocks/block_banner.png");
-}
-.oe_snippet.o_mail_block_header_social .oe_snippet_thumbnail_img {
-    background-image: url("/mass_mailing/static/src/img/blocks/block_header_social.png");
-}
-.oe_snippet.o_mail_block_header_text_social .oe_snippet_thumbnail_img {
-    background-image: url("/mass_mailing/static/src/img/blocks/block_header_text_social.png");
-}
-.oe_snippet.o_mail_block_text_image .oe_snippet_thumbnail_img {
-    background-image: url("/mass_mailing/static/src/img/blocks/block_text-image.png");
-}
-.oe_snippet.o_mail_block_footer_social .oe_snippet_thumbnail_img {
-    background-image: url("/mass_mailing/static/src/img/blocks/block_footer_social.png");
-}
-.oe_snippet.o_mail_block_footer_seprator .oe_snippet_thumbnail_img {
-    background-image: url("/mass_mailing/static/src/img/blocks/block_footer_seprator.png");
-}
-.oe_snippet.o_mail_block_footer_tag_line .oe_snippet_thumbnail_img {
-    background-image: url("/mass_mailing/static/src/img/blocks/block_footer_tag_line.png");
-}
-.oe_snippet.o_mail_block_two_cols .oe_snippet_thumbnail_img{
-    background-image: url("/mass_mailing/static/src/img/blocks/block_two-column.png");
-}
-.oe_snippet.o_mail_block_title_text .oe_snippet_thumbnail_img{
-    background-image: url("/mass_mailing/static/src/img/blocks/block_title_text.png")
-}
-.oe_snippet.o_mail_block_title_sub .oe_snippet_thumbnail_img{
-    background-image: url("/mass_mailing/static/src/img/blocks/block_title.png")
-}
-.oe_snippet.o_mail_block_image_text .oe_snippet_thumbnail_img{
-    background-image: url("/mass_mailing/static/src/img/blocks/block_image-text.png")
-}
-.oe_snippet.o_mail_block_table_content .oe_snippet_thumbnail_img{
-    background-image: url("/mass_mailing/static/src/img/blocks/block_table.png")
-}
-.oe_snippet.o_mail_block_discount1 .oe_snippet_thumbnail_img{
-    background-image: url("/mass_mailing/static/src/img/blocks/block_discount.png")
-}
-.oe_snippet.o_mail_block_discount2 .oe_snippet_thumbnail_img{
-    background-image: url("/mass_mailing/static/src/img/blocks/block_discount2.png")
-}
-.oe_snippet.o_mail_block_event .oe_snippet_thumbnail_img{
-    background-image: url("/mass_mailing/static/src/img/blocks/block_event.png")
-}
-.oe_snippet.o_mail_block_image .oe_snippet_thumbnail_img{
-    background-image: url("/mass_mailing/static/src/img/blocks/block_image_text.png")
-}
-.oe_snippet.o_mail_block_three_cols .oe_snippet_thumbnail_img{
-    background-image: url("/mass_mailing/static/src/img/blocks/block_three_columns.png")
-}
-.oe_snippet.o_mail_block_steps .oe_snippet_thumbnail_img{
-    background-image: url("/mass_mailing/static/src/img/blocks/block_steps.png")
-}
-
-/* ---- template ----  */
-
-.oe_snippet.o_mail_block_odoo_template .oe_snippet_thumbnail_img{
-    background-image: url("/mass_mailing/static/src/img/theme_imgs/odoo_template_thumb.png")
-}
-.oe_snippet.o_mail_block_default_template .oe_snippet_thumbnail_img{
-    background-image: url("/mass_mailing/static/src/img/theme_imgs/default_template_thumb.png")
-}
-.oe_snippet.o_mail_block_airmail_template .oe_snippet_thumbnail_img{
-    background-image: url("/mass_mailing/static/src/img/theme_imgs/airmail_template_thumb.png")
-}
-.oe_snippet.o_mail_block_lemon_template .oe_snippet_thumbnail_img{
-    background-image: url("/mass_mailing/static/src/img/theme_imgs/lemon_template_thumb.png")
-}
-.oe_snippet.o_mail_block_tech_template .oe_snippet_thumbnail_img{
-    background-image: url("/mass_mailing/static/src/img/theme_imgs/tech_template_thumb.png")
-}

addons/mass_mailing/views/editor_field_html.xml

@@ -9,7 +9,6 @@
             <link rel="stylesheet" href="/mass_mailing/static/src/css/airmail_template.css" type="text/css"/>
             <link rel="stylesheet" href="/mass_mailing/static/src/css/lemon_template.css" type="text/css"/>
             <link rel="stylesheet" href="/mass_mailing/static/src/css/tech_template.css" type="text/css"/> -->
-            <link rel="stylesheet" href="/mass_mailing/static/src/css/mass_mailing_editor_thumb.css" type="text/css"/>
             <script type="text/javascript" src="/mass_mailing/static/src/js/mass_mailing_editor.js"></script>
         </t>
     </t>
@@ -21,7 +20,6 @@
 
 <template id="FieldTextHtmlPopupContent" name="Editor layout">
     <t t-set="head">
-        <link rel="stylesheet" href="/mass_mailing/static/src/css/mass_mailing_editor_thumb.css" type="text/css"/>
         <link rel="stylesheet" href="/mass_mailing/static/src/css/mass_mailing_popup.css" type="text/css"/>
      </t>
     <t t-call="web_editor.layout">