Implement POST /users/search endpoint for APIv4 (#5822)

* Implement POST /users/search endpoint for APIv4

* PLT-2713 Added store functions for searching users that don't have a team

* PLT-2713 Added 'without_team' option when searching users

* PLT-2713 Added 'without_team' option when searching users (v4)

store/store.go

@@ -201,6 +201,7 @@ type UserStore interface {
 	Search(teamId string, term string, options map[string]bool) StoreChannel
 	SearchInChannel(channelId string, term string, options map[string]bool) StoreChannel
 	SearchNotInChannel(teamId string, channelId string, term string, options map[string]bool) StoreChannel
+	SearchWithoutTeam(term string, options map[string]bool) StoreChannel
 	AnalyticsGetInactiveUsersCount() StoreChannel
 	AnalyticsGetSystemAdminCount() StoreChannel
 }