Update religious_events.txt

These are less significant changes than my edits to the "job_lord_spiritual.txt" file, but here's a documentation nonetheless:

Added a 0.8 modifier to any conversions happening before 900.

East Slavic cultures are slightly more likely to convert to Orthodox. (The reasoning is that while the West [Scandinavia, Britain, Poland etc.] converts from Norse to Catholic at a correct pace, the Russian lands seem to go Orthodox very slowly in comparison.

Characters with theological education are less likely to convert now due to their religious indoctrination.

Piety change for conversion to organised religion changed from +250 to -100. This is a "what were they thinking?"-kind of change: after all, if you are willing to abandon your faith for another one, especially for pragmatic reasons, you are not a very pious kind of person, and besides - most other religion changes (like "convert to your capital's religion") also deduct piety.

Removed "Tweaked by Battle bunny" comments - they're no longer necessary now that I'm on here.