PLT-1378 Initial version of emoji reactions (#4520)

* Refactored emoji.json to support multiple aliases and emoji categories

* Added custom category to emoji.jsx and stabilized all fields

* Removed conflicting aliases for :mattermost: and :ca:

* fixup after store changes

* Added emoji reactions

* Removed reactions for an emoji when that emoji is deleted

* Fixed incorrect test case

* Renamed ReactionList to ReactionListView

* Fixed 👍 and 👎 not showing up as possible reactions

* Removed text emoticons from emoji reaction autocomplete

* Changed emoji reactions to be sorted by the order that they were first created

* Set a maximum number of listeners for the ReactionStore

* Removed unused code from Textbox component

* Fixed reaction permissions

* Changed error code when trying to modify reactions for another user

* Fixed merge conflicts

* Properly applied theme colours to reactions

* Fixed ESLint and gofmt errors

* Fixed ReactionListContainer to properly update when its post prop changes

* Removed unnecessary escape characters from reaction regexes

* Shared reaction message pattern between CreatePost and CreateComment

* Removed an unnecessary select query when saving a reaction

* Changed reactions route to be under /reactions

* Fixed copyright dates on newly added files

* Removed debug code that prevented all unit tests from being ran

* Cleaned up unnecessary code for reactions

* Renamed ReactionStore.List to ReactionStore.GetForPost

webapp/sass/layout/_post.scss

@@ -1206,3 +1206,31 @@
     margin-left: 50px !important;
     min-width: 320px;
 }
+
+.post-reaction-list {
+    height: 24px;
+}
+
+.post-reaction {
+    border: 1px solid $primary-color;
+    border-radius: 3px;
+    cursor: pointer;
+    display: inline-block;
+    padding: 1px 2px;
+    @include user-select(none);
+
+    .post-reaction__emoji {
+        height: 14px;
+        margin-top: 3px;
+        width: 14px;
+        vertical-align: top;
+    }
+
+    & + & {
+        margin-left: 5px;
+    }
+
+    &--current-user {
+        // background-colour set by theme code
+    }
+}