User management improvements (#1101)

* Show more descriptive success messages for User actions

* Check username uniqueness when creating/updating User

* Adjust translations

* Add tests for `validateUsernameUnique()`

Co-authored-by: Deluan <deluan@navidrome.org>

ui/src/i18n/en.json

@@ -94,6 +94,11 @@
       },
       "helperTexts": {
         "name": "Changes to your name will only be reflected on next login"
+      },
+      "notifications": {
+        "created": "User created",
+        "updated": "User updated",
+        "deleted": "User deleted"
       }
     },
     "player": {
@@ -167,7 +172,8 @@
       "number": "Must be a number",
       "email": "Must be a valid email",
       "oneOf": "Must be one of: %{options}",
-      "regex": "Must match a specific format (regexp): %{pattern}"
+      "regex": "Must match a specific format (regexp): %{pattern}",
+      "unique": "Must be unique"
     },
     "action": {
       "add_filter": "Add filter",