Implement a few channel member endpoints for APIv4 (#5304)

* Implement GET /channels/{channel_id}/members

* Implement GET /channels/{channel_id}/members/{user_id} endpoint for APIv4

* Implement /users/{user_id}/teams/{team_id}/channels/members endpoint for APIv4

* Fix unit test

api4/api.go

@@ -38,7 +38,7 @@ type Routes struct {
 	ChannelsForTeam       *mux.Router // 'api/v4/teams/{team_id:[A-Za-z0-9]+}/channels'
 	ChannelMembers        *mux.Router // 'api/v4/channels/{channel_id:[A-Za-z0-9]+}/members'
 	ChannelMember         *mux.Router // 'api/v4/channels/{channel_id:[A-Za-z0-9]+}/members/{user_id:[A-Za-z0-9]+}'
-	ChannelMembersForUser *mux.Router // 'api/v4/users/{user_id:[A-Za-z0-9]+}/channels/members'
+	ChannelMembersForUser *mux.Router // 'api/v4/users/{user_id:[A-Za-z0-9]+}/teams/{team_id:[A-Za-z0-9]+}/channels/members'
 
 	Posts           *mux.Router // 'api/v4/posts'
 	Post            *mux.Router // 'api/v4/posts/{post_id:[A-Za-z0-9]+}'
@@ -106,7 +106,7 @@ func InitApi(full bool) {
 	BaseRoutes.ChannelsForTeam = BaseRoutes.Team.PathPrefix("/channels").Subrouter()
 	BaseRoutes.ChannelMembers = BaseRoutes.Channel.PathPrefix("/members").Subrouter()
 	BaseRoutes.ChannelMember = BaseRoutes.ChannelMembers.PathPrefix("/{user_id:[A-Za-z0-9]+}").Subrouter()
-	BaseRoutes.ChannelMembersForUser = BaseRoutes.User.PathPrefix("/channels/members").Subrouter()
+	BaseRoutes.ChannelMembersForUser = BaseRoutes.User.PathPrefix("/teams/{team_id:[A-Za-z0-9]+}/channels/members").Subrouter()
 
 	BaseRoutes.Posts = BaseRoutes.ApiRoot.PathPrefix("/posts").Subrouter()
 	BaseRoutes.Post = BaseRoutes.Posts.PathPrefix("/{post_id:[A-Za-z0-9]+}").Subrouter()