emails now only send by category
default category set in INI is 'all', which is special-cased
all other emails send only if their category is active

 we can use this turn on and off email types per-event
WIP email previewing
for viewing emails that are supposed to go out when SEND_EMAILS is False
mostly for debugging but could later be refactored into an email 'queue' system
Merge branch 'ftr_email_categories' into ftr_email_previews
email previewing, merged with email category branch
for viewing emails that are supposed to go out when SEND_EMAILS is False
fix dumb copy+paste error, this now works.
fix bug where AWS_ACCESS_KEY_ID should have been AWS_ACCESS_KEY
tested and confirmed this works.
Merge pull request #278 from EliAndrewC/ftr_email_previews

email previews + Remindercategories