[FIX] mass_mailing: no hover effect on buttons

Most e-mail clients do not support CSS rules for hover effects.
Previously, the mass_mailing application allowed to add buttons with
such effects. This was confusing to some and introduced discrepancies
between the editor preview and the actual e-mails.

This commit removes such hover effects from buttons in the context of
mass mailing.

Task-2936682

closes odoo/odoo#125268

X-original-commit: 55aff2dcdf5537e16bdaf01ee68d776510572171
Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>

addons/web_editor/static/src/scss/web_editor.common.scss

@@ -767,6 +767,16 @@ blockquote {
             .btn-outline-primary {
                 @include button-outline-variant($-btn-primary-border-color);
             }
+            @at-root .o_mail_wrapper #{&}, .o_mail_wrapper {
+                // This requires variable definitions that are only available
+                // here in web_editor (not in mass_mailing).
+                .btn-fill-primary, .btn-primary {
+                    @include button-variant($-btn-primary-color, $-btn-primary-border-color, $-btn-primary-color, $-btn-primary-border-color, $-btn-primary-color, $-btn-primary-border-color);
+                }
+                .btn-outline-primary {
+                    @include button-outline-variant($-btn-primary-border-color, $-btn-primary-border-color, transparent);
+                }
+            }
 
             // Secondary
             $-btn-secondary-color: if($-btn-secondary, o-color($-btn-secondary), map-get($theme-colors, 'secondary'));
@@ -777,6 +787,24 @@ blockquote {
             .btn-outline-secondary {
                 @include button-outline-variant($-btn-secondary-border-color);
             }
+            @at-root .o_mail_wrapper #{&}, .o_mail_wrapper {
+                // This requires variable definitions that are only available
+                // here in web_editor (not in mass_mailing).
+                .btn-fill-secondary, .btn-secondary {
+                    @include button-variant($-btn-secondary-color, $-btn-secondary-border-color, $-btn-secondary-color, $-btn-secondary-border-color, $-btn-secondary-color, $-btn-secondary-border-color);
+                }
+                .btn-outline-secondary {
+                    @include button-outline-variant($-btn-secondary-border-color, $-btn-secondary-border-color, transparent);
+                }
+                .btn-custom:hover,
+                .btn-fill-custom:hover {
+                    filter: invert(0);
+                }
+                .btn-outline-custom:hover {
+                    background-color: transparent !important;
+                    background-image: none !important;
+                }
+            }
 
             // 'Active' states. Note: this only emulates very common components
             // used in snippets. This might need to be more complex the day we