feat: make datatracker_person_id unique

[rudimatz]

fixes #1283

make field datatracker_person_id unique

[jennifer-richards]

Capturing a brief discussion: imo this would be a good change. Before committing to it, though, we need to think a bit more about how we'll deal with datatracker person merges. The code this PR changes was speculatively written to allow for merges to be handled by pointing multiple DatatrackerPerson records at the same datatracker_id. It's not clear if we'll actually want to do that, though.

ietf-tools/datatracker#10558 has a proposal to add multiple IDs per datatracker Person so that after a merge, the UUIDs associated with either of the pre-merge Person records are associated with the post-merge Person. This, or a similar approach, might avoid purple needing to update its records after a merge. (It may not, though, and need to work through the issue before drawing a firm conclusion.)