[REF] mass_mailing: change/add snippets/themes layout/design

addons/mass_mailing/__manifest__.py

addons/mass_mailing/controllers/web_editor.py

addons/mass_mailing/static/src/css/airmail_template.css

addons/mass_mailing/static/src/css/lemon_template.css

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

addons/mass_mailing/static/src/css/odoo_template.css

addons/mass_mailing/static/src/css/snippet_styles.css

addons/mass_mailing/static/src/css/tech_template.css

addons/mass_mailing/static/src/img/blocks/block_banner.png

addons/mass_mailing/static/src/img/blocks/block_button.png

addons/mass_mailing/static/src/img/blocks/block_discount.png

...ns/mass_mailing/static/src/img/blocks/block_discount2.png

addons/mass_mailing/static/src/img/blocks/block_event.png

..._mailing/static/src/img/blocks/block_footer_copyright.png

...s_mailing/static/src/img/blocks/block_footer_seprator.png

...ass_mailing/static/src/img/blocks/block_footer_social.png

...s_mailing/static/src/img/blocks/block_footer_tag_line.png

.../mass_mailing/static/src/img/blocks/block_footer_text.png

...mass_mailing/static/src/img/blocks/block_four_columns.png

...ailing/static/src/img/blocks/block_header_center_logo.png