Backwardly-extensible history in bridged portal rooms (SPEC-440)

It would be lovely if bridged portal rooms that come from some bridged universe that supports history fetching (e.g. Gitter or Slack) could themselves be represented by lazy history backfill inside Matrix, matching the way that individual Matrix HSes can lazily backfill history from other HSes.

There's an annoying asymmetry in that native Matrix rooms can be lazily backfilled over server-to-server federation, but there isn't a similar ability for portal rooms bridged from a 3PN.

(Imported from https://matrix.org/jira/browse/SPEC-440)

(Reported by @leonerd)

Start by reviewing the imported SPEC-440 request and the existing Matrix lazy-backfill model described in the issue. Compare that behavior with bridged portal rooms from history-capable systems such as Gitter or Slack. Done would be a sufficiently specified protocol approach for making that bridged history available through lazy backfill.

Written by the indexing model from the issue text.