Support existing history in bridged Gitter rooms on Matrix

Support existing history in bridged Gitter rooms on Matrix.

There is currently MSC2716: Incrementally importing history into existing rooms which adds this functionality for Application Service (AS) bridges to support existing history. We need to wait until this goes golden in the spec.

• Import all messages
  • !2313 (merged)
  • https://github.com/matrix-org/matrix-hosted/issues/6793
  • https://gitlab.com/gitterHQ/gitter-infrastructure/-/issues/258
• Sync room membership
  • #2857 (closed)
• Sync admin permissions
  • #2860 (closed)
  • #2857 (closed)
• Point "live" room back at historical Matrix room
  • #2858 (closed)