Add an index to the EntryVersion "url" column

[ryanfb]

After #51 (adding a limit to the version select statement), I noticed that this query was still taking quite a large portion of the runtime for diffengine instances with large databases. As it turns out, adding an index to the EntryVersion url column makes this query (and diffengine as a whole) run much faster. This change both sets an index in the peewee model, and runs a migration to try to add an index to any existing database (adapted from this StackOverflow answer).

[edsu]

Lovely, thanks @ryanfb ... and thank for pushing diffengine to its limits, and past them :-)

[edsu]

PS. How did you notice that specific query was taking some time. Was it an educated guess or did you do some fancy profiling tricks?

[ryanfb]

Using py-spy to attach to a running instance, I could see where the time was being spent - specifically, the flame graph showed the call stack for what statements were leading to the execute_sql call that was taking almost 100% of the time.

[edsu]

Ahh, very nice ... thanks for the tip!