Option to automatically purge rooms with no local members

*This issue was originally created by [**@bones-was-here**](https://github.com/bones-was-here) at .*

**Description:**

Feature request: a setting to delete data for rooms the server no longer participates in (all local users have left and/or forgotten the room). Now that #2662 has merged it would be possible to duct tape this on with scripts and cron jobs, but since many admins will need this it would be best if it could be enabled in dendrite.yaml.

Start by reviewing the behavior enabled by #2662 and the existing dendrite.yaml configuration structure. Trace how rooms with no local members or forgotten rooms are represented, then define what data the setting should purge and verify the result with the project's relevant tests or a local server run.

Written by the indexing model from the issue text.