fix db downgrade issues with production snapshots

Updated the 0.17 downgrades to deal with the two possible unique
constraint names, uc_user_id or user_id.

Removed the test for 20140109 entirely because that snapshot was
a mixture of 0.14 and 0.15, due to the postponed 0.15 upgrade. It's not
worth making our automatic migrations handle that mess.

Change-Id: I4706c231d8ae7c9efb4a0817d2efc12ce48f61f8