Rather than relying on `outlier` being in the `internal_metadata` column,
populate it based on the `events.outlier` column.

Ultimately, this will allow us to stop writing it to the database. For now, we
have to grandfather it back in so as to maintain compatibility with older
versions of Synapse.

it turns out we *do* send outliers this way, in the form of locally-generated
invite rejections.

this isn't making it into Synapse 1.30.