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/params.go

@@ -26,6 +26,7 @@ type ApiParams struct {
 	HookId    string
 	EmojiId   string
 	Email     string
+	Username  string
 	Page      int
 	PerPage   int
 }
@@ -71,6 +72,10 @@ func ApiParamsFromRequest(r *http.Request) *ApiParams {
 		params.Email = val
 	}
 
+	if val, ok := props["username"]; ok {
+		params.Username = val
+	}
+
 	if val, err := strconv.Atoi(r.URL.Query().Get("page")); err != nil {
 		params.Page = PAGE_DEFAULT
 	} else {