[FIX] mass_mailing: fix fullscreen icon size

Fix the email designer fullscreen icon size by adding the .img-fluid css
class to make the icon image responsive and ensuring it won't scale
beyond its parents.

For screens of max-width 1135px, a css rule applies a 100% min-width
on every only child image which doesn't have the img-fluid class.
This min-width rule was making the icon size bigger than it's actual parent.
Setting the img-fluid class on the image icon for it to not match this rule
fixed the issue.

Task-3604699

closes odoo/odoo#143831

X-original-commit: f5be98b29ac1efb9f547e38d0fde4e65dcc5b8a6
Signed-off-by: Warnon Aurélien (awa) <awa@odoo.com>
Signed-off-by: Amélie Dieudonné (amdi) <amdi@odoo.com>

addons/mass_mailing/views/snippets_themes.xml

@@ -11,7 +11,7 @@
                 <i class="fa fa-lg fa-mobile"/>
             </button>
             <button class="o_fullscreen_btn btn btn-primary">
-                <img src="/web_editor/font_to_img/61541/rgb(255,255,255)/16" alt="Fullscreen"/>
+                <img class="img-fluid" src="/web_editor/font_to_img/61541/rgb(255,255,255)/16" alt="Fullscreen"/>
             </button>
         </div>
     </xpath>