This adds a new backfill type for media that sends a request to the
phone for every media that is not available on the WA servers. WA
deletes media from their servers after about two weeks, so you have to
ask the phone to re-upload it.
In order to use this, you need to enable
bridge.history_sync.backfill_media and configure the requests that will
be made per portal using bridge.history_sync.media (which is similar to
the deferred backfill config).
If you already have backfilled portals, but want to do a one-off media
backfill for all existing portals, you can set
bridge.history_sync.enqueue_backfill_media_next_start to true.
