UI: Add a %REWARD% entry for briefings

[wheelyjoe]

To allow ticket reward to be printed to the briefing for clarity to players if desired by designer.

[jtoppins]

What value would you want displayed? The resultant tickets after the modifier is applied or just the base cost of the template. If the resultant value is wanted this would require a new API for the tickets class to keep isolation of concerns in check, as currently the number of tickets available and how the modifier is applied is all contained in the Ticket class. And this information does not belong in the briefing generation code.

[wheelyjoe]

Ah right, maybe make this a longer term QOL thing then, it's certainly not important, but would be nice to be available.