R1:76665f725bc9

Correct a bad Herald migration

Summary:
One of the migrations in rPa335004a91 (`20150730.herald.5.sql`) incorrectly swapped "add" and "add blocking" Differential Herald rules.

Swap any rules last modified before this patch was applied back. This is the best we can do without possibly overwriting more recent, intentional data. I'll issue some guidance on this in the changelog.

Test Plan:
- Made a rule, ran patch, no change.
- Changed rule modified time to a few months ago, ran patch, saw swap from…