More customization options for emergency build

**What do you want to see?**
More customization options for emergency build in Settings.txt:
* Allow "emergency build" and "slow build" strings to be modified (e.g. Carrier Battles would set them to something like "savings mode" and "quick build")
* Allow customization of number of slow build turns per emergency build turn and whether this rounds up or down

**How would it help players?**
The customizable strings would make the UI in Carrier Battles more intuitive.
The customizable number of turns would open up a lot of interesting possibilities such as having 10 turns of 200% rate build for 15 turns of 0% rate or whatever.

**Alternatives**
The strings could be customized in a different file, such that we would be able to customize all strings used in the game a la SE5's MainStrings.txt