Automatic train stations (#1353)

## Description:

Train stations are now built automatically when a factory is
constructed.

Changes:
- When a factory is built, nearby structures are connected to the rail
network
- When a city is built near a factory, it is connected to the rail
network
    - All structures behave the same when a train stops: to be defined
    - Removed station badge
    - Gold income is now related to the structure's level

## 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

src/client/SinglePlayerModal.ts

@@ -40,10 +40,7 @@ export class SinglePlayerModal extends LitElement {
   @state() private gameMode: GameMode = GameMode.FFA;
   @state() private teamCount: number | typeof Duos = 2;
 
-  @state() private disabledUnits: UnitType[] = [
-    UnitType.Factory,
-    UnitType.Train,
-  ];
+  @state() private disabledUnits: UnitType[] = [UnitType.Factory];
 
   private userSettings: UserSettings = new UserSettings();