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/layout/_headers.scss

@@ -121,7 +121,7 @@
 
         &.dropdown {
             float: left;
-            max-width: 100%;
+            max-width: 90%;
             padding-right: 1em;
 
             .header-dropdown__icon {
@@ -182,7 +182,6 @@
     }
 
     .channel-intro-profile {
-        margin-left: 63px;
         margin-top: 5px;
 
         .user-popover {
@@ -193,7 +192,10 @@
     }
 
     .channel-intro-img {
-        float: left;
+        .status-wrapper {
+            height: 50px;
+            margin: 0 10px 10px 0;
+        }
 
         img {
             @include border-radius(100px);
@@ -211,10 +213,6 @@
         background: $bg--gray;
         padding: 10px 15px;
     }
-
-    .channel-intro-text {
-        margin-top: 35px;
-    }
 }
 
 // Team Header in Sidebar