Add an ursabot option to force re-run benchmarks

[ianmcook]

Currently if you do @ursabot please benchmark and the last commit has already been benchmarked then ursabot replies with Commit xxxxxxx already has scheduled benchmark runs. This is a good default, but it would be nice to have an option to force re-run the benchmarks on the same commit, for example if you suspect that a regression might just be noise.

[austin3dickey]

I think this will require us to stop using the commit hash as the primary key of the Benchmarkable table. If we put the commit string that we currently use into a different column, then we could have multiple Benchmarkable entries with different primary keys that refer to the same commit. This will require careful testing to ensure we don't break things.

[alistaire47]

I messed with this a while ago, and there are some strong and not great assumptions about run_id that messing with this will likely violate, so this may turn into a bit of a mess that has to touch quite a lot of the repo