[FIX] mail: chatter has horizontal scroll on medium screen sizes

A horizontal scroll appears on medium to large screen sizes in the
form's chatter sidebar (e.g. between 1534px and 1619px, when all modules
are installed).

This commit restores the padding set by Bootstrap for `.nav-link`
elements instead of a fraction of the grid gutter's (?) width then
applying slightly smaller padding.

Task ID: 2193950

X-original-commit: 7bc8b577d822b057d1bccf881b9c4b1ee127a728

addons/mail/static/src/scss/chatter.scss

@@ -11,7 +11,7 @@
         .btn {
             @extend .text-nowrap;
             margin-bottom: -$nav-tabs-border-width;
-            padding: $grid-gutter-width*0.25 - $nav-tabs-border-width $grid-gutter-width*0.5 $grid-gutter-width*0.25;
+            padding: $nav-link-padding-y $nav-link-padding-x;
             border-radius: $border-radius-sm $border-radius-sm 0 0;
         }