Don't allow duplicates in the bugs field.

It's been possible for a long time to put in the same bug number twice. In the
past, this caused all sorts of nonsense (making it impossible to remove just
one of them), but with the fields rewrite the situation is much improved,
just being ugly.

I've added a filter to get_bug_list() which uses set() to de-dupe the list.
Theoretically it's still possible that a duplicate gets saved in the draft, but
it will never be shown to the user, and it will get filtered out when saving
the draft.

Testing done:
Tried to save the "Bugs" field with the same bug listed multiple times. Saw
that all my duplicates were filtered out.

Fixes bug 3063.

Reviewed at https://reviews.reviewboard.org/r/5358/