PLT-3145 Custom Emojis (#3381)

* Reorganized Backstage code to use a view controller and separated it from integrations code

* Renamed InstalledIntegrations component to BackstageList

* Added EmojiList page

* Added AddEmoji page

* Added custom emoji to autocomplete and text formatter

* Moved system emoji to EmojiStore

* Stopped trying to get emoji before logging in

* Rerender posts when emojis change

* Fixed submit handler on backstage pages to properly support enter

* Removed debugging code

* Updated javascript driver

* Fixed unit tests

* Fixed backstage routes

* Added clientside validation to prevent users from creating an emoji with the same name as a system one

* Fixed AddEmoji page to properly redirect when an emoji is created successfully

* Fixed updating emoji list when an emoji is deleted

* Added type prop to BackstageList to properly support using a table for the list

* Added help text to EmojiList

* Fixed backstage on smaller screen sizes

* Disable custom emoji by default

* Improved restrictions on creating emojis

* Fixed non-admin users seeing the option to delete each other's emojis

* Fixing gofmt

* Fixed emoji unit tests

* Fixed trying to get emoji from the server when it's disabled

webapp/sass/responsive/_mobile.scss

@@ -19,7 +19,6 @@
         display: block;
 
         .backstage-filter__search {
-            border-bottom: 1px solid $light-gray;
             margin: 10px 0;
             width: 100%;
         }