add api endpoint for commitment merging

[Varsius]

Checklist:

• I updated the documentation to describe the semantical or interface changes I introduced.

[coveralls]

coverage: 79.465% (+0.04%) from 79.422%
when pulling 614d361 on commitment-merging
into 7cc255c on master.

[majewsky]

I will do a more detailed review later today or tomorrow, but for the time being, here are comments on the DB migration and model:

[majewsky]

FYI, I read a bit more (didn't get to finish again....). To make the feedback cycle shorter, I added some commits for minor stuff. Please review my commits, esp. the one touching the migrations.

[majewsky]

We are nearing completion here. I pushed another commit with just whitespace changes (esp. in tr.DBChanges().AssertEqualf(); that fellow normalizes whitespace on its input, so you can make liberal use of indentation to make the test code look neat).

Finally, since #667 was merged, you will need to do a rebase. There will be a few merge conflicts.

[majewsky]

When you merge this, please take a look at the qa-de-1 database to see how the DB migration goes.