Add opt_out column to expt_assignments table

[karlhigley]

This also renames the table from expt_allocations to expt_assignments. That felt more urgent now that we're going to have additional tables (like datasets and account_aliases) that will connect to related tables like accounts.

[kluver]

we should figure out what we want to do with the code style failures, but the code itself seems straightforward and good.

[kluver]

I was hoping to downstream some changes from this PR (so I didn't have to deal with the alembic merge) -- I don't think this passes the database tests.

[kluver]

I was hoping to downstream some changes from this PR (so I didn't have to deal with the alembic merge) -- I don't think this passes the database tests.

    if isinstance(revision.down_revision, tuple):
        return  # don't test merge revisions

Adding this to the staircase test "fixes" it -- it does prevent testing merge revisions, but I think we can plan to always have empty merge revisions