Note about invalid constraint
Handle missing activity entries
Correct base issue plugin redirect
Add custom csrf failure error page
Various tweaks to error pages
Kill possible recursion on csrf decorator
Relabel events as samples
Support INT and BIGINT foreignkeys

- Add SENTRY_USE_BIG_INTS (defaults to True on new installs)
- Add FlexibleForeignKey which correctly uses the related fields lookup type
- Clean up various migrations so they are correct based on assumed schema

Refs GH-1354
Merge remote-tracking branch 'upstream/master'