Add Migration for efficient rollback

[kderme]

Fixes #256 and maybe #397. There are two persistent issues that needs to be fixed before merging this

• One has a pr Fix references Migrations yesodweb/persistent#1167
• The other I haven't fixed yet. The generated migration is slightly wrong since it drops a constrant:
  EXECUTE 'ALTER TABLE "block" DROP CONSTRAINT "block_previous_id_fkey"' ;
  without replacing it with a correct one. It looks like a persistent bug, easy to fix.

[erikd]

This is great, but I wonder if it might be more sensible to wait until these are formally released.

[kderme]

The second bug is discussed here and a fix is proposed. The fix seems to work at least for our case, so this pr is ready to test now. My manual tests show that slot rollback works fine.

This is great, but I wonder if it might be more sensible to wait until these are formally released.

Yes my fixes will probably change by the time they're merged.

[erikd]

Probably worth investigating these CI failures.

[kderme]

Probably worth investigating these CI failures.

This seems to be related to the --only-dependencies flag of ci and the fact that I use a custom branch of persistent at the cabal.project. Eventually though we won't have a custom branch (unless we want this pr merged asap).

[erikd]

Rebased and squashed down this PR. Hope it passes CI.