[FIX] web: button assign appear on mouse hover in Activity View

Steps:
 - Open Projects
 - Go to Tasks
 - Open Activity view
 - There is a button to assign a new user

 Issue:
 - The button to assign a new user should only be visible on hover

 Cause:
 - There was no css class added for button in activity record.

 Fix:
 - By making button visibility hidden by default and should be visible only on
   hover.

closes odoo/odoo#142381

Task: 3522113
X-original-commit: d60519ae4cb3c9c7d83e8143c3015c28b2a929ad
Signed-off-by: Luca Vitali (luvi) <luvi@odoo.com>

addons/web/static/src/views/fields/many2many_tags_avatar/many2many_tags_avatar_field.scss

@@ -19,7 +19,7 @@
 
 }
 
-.o_kanban_record  {
+.o_kanban_record, .o_activity_record  {
     @include media-breakpoint-up(sm) {
         .o_field_widget.o_field_many2many_tags_avatar .o_quick_assign {
             visibility: hidden;