Implement GET /users/username/{username} endpoint for APIv4 (#5310)

* added get user by username endpoint

* added get user by username unit test and driver

* changed username length to 22 characters max

* changed Params to UserName to Username

* reorganized get user by username and get user by email formatting in model/client4

api4/api.go

@@ -90,7 +90,7 @@ func InitApi(full bool) {
 
 	BaseRoutes.Users = BaseRoutes.ApiRoot.PathPrefix("/users").Subrouter()
 	BaseRoutes.User = BaseRoutes.Users.PathPrefix("/{user_id:[A-Za-z0-9]+}").Subrouter()
-	BaseRoutes.UserByUsername = BaseRoutes.Users.PathPrefix("/username/{username:[A-Za-z0-9_-.]+}").Subrouter()
+	BaseRoutes.UserByUsername = BaseRoutes.Users.PathPrefix("/username/{username:[A-Za-z0-9\\_\\-\\.]+}").Subrouter()
 	BaseRoutes.UserByEmail = BaseRoutes.Users.PathPrefix("/email/{email}").Subrouter()
 
 	BaseRoutes.Teams = BaseRoutes.ApiRoot.PathPrefix("/teams").Subrouter()