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/utilities/RenderUnitTypeOptions.ts

@@ -18,7 +18,6 @@ const unitOptions: { type: UnitType; translationKey: string }[] = [
   { type: UnitType.AtomBomb, translationKey: "unit_type.atom_bomb" },
   { type: UnitType.HydrogenBomb, translationKey: "unit_type.hydrogen_bomb" },
   { type: UnitType.MIRV, translationKey: "unit_type.mirv" },
-  { type: UnitType.Train, translationKey: "unit_type.train" },
   { type: UnitType.Factory, translationKey: "unit_type.factory" },
 ];