Implement some channel endpoints for APIv4 (#5767)

api4/api.go

@@ -35,6 +35,7 @@ type Routes struct {
 
 	Channels                 *mux.Router // 'api/v4/channels'
 	Channel                  *mux.Router // 'api/v4/channels/{channel_id:[A-Za-z0-9]+}'
+	ChannelForUser           *mux.Router // 'api/v4/users/{user_id:[A-Za-z0-9]+}/channels/{channel_id:[A-Za-z0-9]+}'
 	ChannelByName            *mux.Router // 'api/v4/teams/{team_id:[A-Za-z0-9]+}/channels/name/{channel_name:[A-Za-z0-9_-]+}'
 	ChannelByNameForTeamName *mux.Router // 'api/v4/teams/name/{team_name:[A-Za-z0-9_-]+}/channels/name/{channel_name:[A-Za-z0-9_-]+}'
 	ChannelsForTeam          *mux.Router // 'api/v4/teams/{team_id:[A-Za-z0-9]+}/channels'
@@ -113,6 +114,7 @@ func InitApi(full bool) {
 
 	BaseRoutes.Channels = BaseRoutes.ApiRoot.PathPrefix("/channels").Subrouter()
 	BaseRoutes.Channel = BaseRoutes.Channels.PathPrefix("/{channel_id:[A-Za-z0-9]+}").Subrouter()
+	BaseRoutes.ChannelForUser = BaseRoutes.User.PathPrefix("/channels/{channel_id:[A-Za-z0-9]+}").Subrouter()
 	BaseRoutes.ChannelByName = BaseRoutes.Team.PathPrefix("/channels/name/{channel_name:[A-Za-z0-9_-]+}").Subrouter()
 	BaseRoutes.ChannelByNameForTeamName = BaseRoutes.TeamByName.PathPrefix("/channels/name/{channel_name:[A-Za-z0-9_-]+}").Subrouter()
 	BaseRoutes.ChannelsForTeam = BaseRoutes.Team.PathPrefix("/channels").Subrouter()