Factory spawns trains (#1408)

## Description:

- Change trains so it spawns from factories only
- Increase train frequency as they will now spawn from a single
structure.
- Factory will spawn more trains depending on its level
- Fix port to connect to nearby railroads
- Add factory description

## 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

---------

Co-authored-by: Tom Rouillard <trouilla@mathworks.com>
Co-authored-by: evanpelle <evanpelle@gmail.com>

src/client/styles.css

@@ -369,6 +369,11 @@ label.option-card:hover {
   mask: url("../../resources/images/CityIconWhite.svg") no-repeat center / cover;
 }
 
+#helpModal .factory-icon {
+  mask: url("../../resources/images/FactoryIconWhite.svg") no-repeat center /
+    cover;
+}
+
 #helpModal .defense-post-icon {
   mask: url("../../resources/images/ShieldIconWhite.svg") no-repeat center /
     cover;