feat(rollups): add rollup_contents mapping

[Carbonhell]

Closes #494.

I wasn't sure whether to use IDs from the pull_request table to represent PRs in the mapping, or just the GitHub PR number - I went with the latter as there would be no record for the rollup at the time of its creation, as that gets populated via GitHub events, if I understood correctly.

I haven't experienced rollups at all as I haven't (yet?) contributed to a project with bors, but I noticed there's no safety check for multiple rollups containing the same PR(s) being opened. I found a related comment here, but I kept it out of this PR to reduce its scope and simplify review. I think such a safety check could be added easily by checking for rollups associated to each candidate PR, and verifying the status of the rollup PR to ignore closed ones. That would still allow one to recreate the same scenario by reopening a previously closed rollup, though.

One thing I'm unsure about is how to handle the error when writing the rollup_contents records in the DB. I think that should only happen either in case of DB connection errors, or if the list of PRs contains duplicates. I assume the former can be propagated to the caller, whereas I'm not sure if the latter is realistic and should be accounted for. I'm thinking we can just ignore the conflicting records in such case - let me know if I should proceed with that.

Edit: sorry, I forgot the migration test files. Will push them tomorrow as soon as possible

[Kobzol]

Hi, thanks for the PR! I'll take a look, but I don't know when I'll be able to get to it, since it's Holiday season :)

I wasn't sure whether to use IDs from the pull_request table to represent PRs in the mapping, or just the GitHub PR number - I went with the latter as there would be no record for the rollup at the time of its creation, as that gets populated via GitHub events, if I understood correctly.

Yeah, that makes sense.

but I noticed there's no safety check for multiple rollups containing the same PR(s) being opened.

This is not a problem on its own, there are often multiple rollups that contain a single PR, because if a rollup fails, you create a new one, often before the old one is closed. I don't think that we need to think about that right now.

if the list of PRs contains duplicates. I assume the former can be propagated to the caller, whereas I'm not sure if the latter is realistic and should be accounted for.

Also a good point! I'll ignore duplicate PR numbers in rollups.