Adds elasticsearch to the user and channel autocompletion functions (#10354)

* Adds elasticsearch to the user and channel autocompletion functions

* Implement channel store GetChannelsByIds test

* Style changes and govet fixes

* Add gofmt fixes

* Extract default channel search limit to a const

* Add StringSliceDiff function to the utils package

* Honor USER_SEARCH_MAX_LIMIT on the user autocomplete api handler

* Change the elasticsearch development image

einterfaces/elasticsearch.go

@@ -15,6 +15,13 @@ type ElasticsearchInterface interface {
 	IndexPost(post *model.Post, teamId string) *model.AppError
 	SearchPosts(channels *model.ChannelList, searchParams []*model.SearchParams, page, perPage int) ([]string, model.PostSearchMatches, *model.AppError)
 	DeletePost(post *model.Post) *model.AppError
+	IndexChannel(channel *model.Channel) *model.AppError
+	SearchChannels(teamId, term string) ([]string, *model.AppError)
+	DeleteChannel(channel *model.Channel) *model.AppError
+	IndexUser(user *model.User, teamsIds, channelsIds []string) *model.AppError
+	SearchUsersInChannel(teamId, channelId, term string, options *model.UserSearchOptions) ([]string, []string, *model.AppError)
+	SearchUsersInTeam(teamId, term string, options *model.UserSearchOptions) ([]string, *model.AppError)
+	DeleteUser(user *model.User) *model.AppError
 	TestConfig(cfg *model.Config) *model.AppError
 	PurgeIndexes() *model.AppError
 	DataRetentionDeleteIndexes(cutoff time.Time) *model.AppError