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

einterfaces/emoji.go

@@ -0,0 +1,22 @@
+// Copyright (c) 2016 Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package einterfaces
+
+import (
+	"github.com/mattermost/platform/model"
+)
+
+type EmojiInterface interface {
+	CanUserCreateEmoji(string, []*model.TeamMember) bool
+}
+
+var theEmojiInterface EmojiInterface
+
+func RegisterEmojiInterface(newInterface EmojiInterface) {
+	theEmojiInterface = newInterface
+}
+
+func GetEmojiInterface() EmojiInterface {
+	return theEmojiInterface
+}