PLT-2559 Always return successful when trying to join a channel that the user is already a member of (#3265)

* Added unit tests for SqlChannelStore.GetMember

* Fixed api routes for accessing channels by name when the name includes an underscore

* Changed join channel API to always return successful when the user is already a member of the channel

api/api.go

@@ -25,7 +25,7 @@ type Routes struct {
 
 	Channels        *mux.Router // 'api/v3/teams/{team_id:[A-Za-z0-9]+}/channels'
 	NeedChannel     *mux.Router // 'api/v3/teams/{team_id:[A-Za-z0-9]+}/channels/{channel_id:[A-Za-z0-9]+}'
-	NeedChannelName *mux.Router // 'api/v3/teams/{team_id:[A-Za-z0-9]+}/channels/name/{channel_name:[A-Za-z0-9-]+}'
+	NeedChannelName *mux.Router // 'api/v3/teams/{team_id:[A-Za-z0-9]+}/channels/name/{channel_name:[A-Za-z0-9_-]+}'
 
 	Posts    *mux.Router // 'api/v3/teams/{team_id:[A-Za-z0-9]+}/channels/{channel_id:[A-Za-z0-9]+}/posts'
 	NeedPost *mux.Router // 'api/v3/teams/{team_id:[A-Za-z0-9]+}/channels/{channel_id:[A-Za-z0-9]+}/posts/{post_id:[A-Za-z0-9]+}'
@@ -60,7 +60,7 @@ func InitApi() {
 	BaseRoutes.NeedTeam = BaseRoutes.Teams.PathPrefix("/{team_id:[A-Za-z0-9]+}").Subrouter()
 	BaseRoutes.Channels = BaseRoutes.NeedTeam.PathPrefix("/channels").Subrouter()
 	BaseRoutes.NeedChannel = BaseRoutes.Channels.PathPrefix("/{channel_id:[A-Za-z0-9]+}").Subrouter()
-	BaseRoutes.NeedChannelName = BaseRoutes.Channels.PathPrefix("/name/{channel_name:[A-Za-z0-9-]+}").Subrouter()
+	BaseRoutes.NeedChannelName = BaseRoutes.Channels.PathPrefix("/name/{channel_name:[A-Za-z0-9_-]+}").Subrouter()
 	BaseRoutes.Posts = BaseRoutes.NeedChannel.PathPrefix("/posts").Subrouter()
 	BaseRoutes.NeedPost = BaseRoutes.Posts.PathPrefix("/{post_id:[A-Za-z0-9]+}").Subrouter()
 	BaseRoutes.Commands = BaseRoutes.NeedTeam.PathPrefix("/commands").Subrouter()