Add (unique) index to forum_member_id

[wilson29thid]

I can't believe it, but we haven't had an index on members.forum_member_id this whole time. Literally every API request in a logged in session does a lookup on that column 😬

While we're at it, we should make it a unique index. But there are two isues with that: there are a bunch of records with the value set to 0, as well as 4 duplicates somehow.

Might be easier to just wait until after the discourse transition, as those values will be reset.