[FIX] web: do not overlap calendar popover

The content of the popover was overflowing on the button if they were
too many elements. It will now display a scrollbar when needed.

TaskID: 2367543

addons/web/static/src/scss/web_calendar.scss

@@ -687,6 +687,8 @@ $o-cw-filter-avatar-size: 20px;
     .o_cw_popover_fields_secondary {
         max-height: 170px; // Fallback for old browsers
         max-height: 25vh;
+        overflow: auto;
+        padding-bottom: 1px; // prevents the scrollbar to show when not needed
 
         &::-webkit-scrollbar {
             background: gray('200');