Store first-RTBC timestamp and add column to issue listings

Opening this here because it's probably too specific for project_issue.

It would be useful when going through the RTBC queue, to know how long a particular issue has been RTBC (or rather, when it was first RTBCed).

I usually order the RTBC queue by least recently updated to ensure issues don't sit there for too long without anyone looking at them, but patch re-tests/re-rolls etc. mean that it's not possible to go through the queue FIFO in any real sense.

Storing the timestamp when an issue is first moved to RTBC would let us add an additional column to the listing. Could also be useful information to track across all projects (i.e. average length of time an issue is at RTBC).

If we wanted to make this more generic, we could store a timestamp for the first move to any status. That could be useful for stats like how long issues go from active -> CNR or CNR -> commit.