Implement RMAU tracking and optionally blocking the bridge after a RMAU limit is reached

This makes us dependent on matrix-appservice-bridge to make use of
UserActivityTracker and BridgeBlocker, which also bumps matrix-bot-sdk
to a higher required version.

Do not use default

Allow admins to disable portal rooms and enforce room limits

Metrics.enable: Correct config sample and schema

Merge branch 'cos/update_sdk_version' of https://github.com/vranki/matrix-appservice-discord into develop

Merge branch 'j94/copy-changes' of https://github.com/jaller94/matrix-appservice-discord into develop

Updated README with Riot->Element and end user documentation