PLT-3077 Add group messaging (#5489)

* Implement server changes for group messaging

* Majority of client-side implementation

* Some server updates

* Added new React multiselect component

* Fix style issues

* Add custom renderer for options

* Fix model test

* Update ENTER functionality for multiselect control

* Remove buttons from multiselect UI control

* Updating group messaging UI (#5524)

* Move filter controls up a component level

* Scroll with arrow keys

* Updating mobile layout for multiselect (#5534)

* Fix race condition when backspacing quickly

* Hidden or new GMs show up for regular messages

* Add overriding of number remaining text

* Add UI filtering for team if config setting set

* Add icon to channel switcher and class prop to status icon

* Minor updates per feedback

* Improving group messaging UI (#5563)

* UX changes per feedback

* Update email for group messages

* UI fixes for group messaging (#5587)

* Fix missing localization string

* Add maximum users message when adding members to GM

* Fix input clearing on Android

* Updating group messaging UI (#5603)

* Updating UI for group messaging (#5604)

webapp/sass/responsive/_mobile.scss

@@ -1,6 +1,23 @@
 @charset 'UTF-8';
 
 @media screen and (max-width: 768px) {
+    .multi-select__container {
+        .btn {
+            display: block;
+            min-width: 50px;
+        }
+
+        .Select-value-label {
+            max-width: 190px;
+        }
+    }
+
+    .more-modal__list {
+        .more-modal__actions--round {
+            @include opacity(.5);
+        }
+    }
+
     .post-create__container {
         padding-bottom: 10px;