PLT-4815 Refactor 'More Channels' modal into the new modal pattern (#4742)

* Refactor 'More Channels' modal into the new modal pattern

* Fix unit test

* Readded CSS changes

webapp/sass/components/_modal.scss

@@ -483,6 +483,7 @@
         .filter-row {
             @include clearfix;
             margin: 5px 0 10px;
+            width: 300px;
         }
 
         .member-count {