Write tests for the DB migrations

We now have a framework to make it easy to write them, and we should just iterate on them one by one.

I hereby propose that every person who wants to write a test claims it here (by commenting "I want to write the test for migration N", where N is the migration's number, as known as the number next to the m function name in the migrations array, numbered from 0), since that's something we can do in parallel to move forward faster.

• m0
• m1
• m2
• m3
• m4
• m5
• m6
• m7
• m8
• m9
• m10
• m11
• m12
• m13
• m14
• m15
• m16
• m17 (repetition of m16)
• m18
• m19
• delete the cache and all now useless modifications like delete op.property (since op is not cached anymore)
• Fix m6 test