[FIX] mail: allow the followers list to be scrolled

The follower's list has an ancestor (the o_chatter_topbar) that has a 33px size.
Thus it has not been given any max-height, so that it can overflow
that 33px limit.
But it also means that it can overflow the screen, without any scrollbar,
so long lists are unreadable and unusable.

We set the max-height to a reasonable value, allowing for scrollbars.

opw 1922062

addons/mail/static/src/scss/followers.scss

@@ -84,6 +84,8 @@
 
         .o_followers_list {
             font-size: small;
+            max-height: 50vh; // ancestor has size 33px
+            overflow: auto;
 
             .o_partner {
                 position: relative; // required for old design compatibility