Mark train stations and factories as experimental (#1309)

## Description:

Train stations and factories won't be ready for v24.
Disable them by default. They can be reactivated on private lobies and
solo games, allowing us to gather feedbacks.

Changes:
- added an "experimental" attribute for units. When set, they are hidden
entirely when disabled, rather than appearing grayed out.
 - disabled train stations and factories by default.

Default values:

![image](https://github.com/user-attachments/assets/ec91ef80-c0ef-45a7-8e6c-6628a836b593)

No factories:

![image](https://github.com/user-attachments/assets/a3e35607-aed5-401c-bc38-4ea742d057c6)


## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [x] I have added relevant tests to the test directory
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced
- [x] I understand that submitting code with bugs that could have been
caught through manual testing blocks releases and new features for all
contributors

## Please put your Discord username so you can be contacted if a bug or
regression is found:

IngloriousTom

resources/lang/en.json

@@ -241,7 +241,9 @@
     "sam_launcher": "SAM Launcher",
     "atom_bomb": "Atom Bomb",
     "hydrogen_bomb": "Hydrogen Bomb",
-    "mirv": "MIRV"
+    "mirv": "MIRV",
+    "train": "Train",
+    "factory": "Factory"
   },
   "user_setting": {
     "title": "User Settings",