SecExp: Remove hasFought global flags

Inspired by comments in !7322 (merged)

hasFoughtBattleOnce, hasFoughtMajorBattleOnce, and hasRebelledOnce are entirely redundant with the global counters (battlesCount, majorBattlesCount, and rebellionsCount), so just use the counters instead of tracking everything twice. Duplicate data is bad data.

Also deduplicates a bulk of the description text between the first and subsequent major battle.