Enable message retention configuration policy.

[brndnmtthws]

This allows people to enable message retention policies on their homeserver.

One note: there are a few extra features that are available, but I decided to err on the side of simplicity in the implementation. For example, purge_jobs takes an array with a bunch of possible parameters, but I just simplified it and stuck with the defaults. If preferred, I can change the settings so that purge_jobs uses an array for the config values instead of extracting the parameters.

[spantaleev]

Thank you! 👍

While this is very nice, I wonder if we should keep introducing more and more dedicated variables for controlling homeserver.yaml. There are hundreds of options there (some more flexible and harder to control than others).

Generally, it's nice if we can keep things simpler (less dedicated options in the playbook) and rely on the universal mechanism we have for extending the configuration - matrix_synapse_configuration_extension_yaml. Using that, anyone can override and inject any configuration key, and we won't have to keep growing the list of variables we define as part of the playbook.

On a related note, retention.enabled is a very unfortunate name for this configuration option (but that's the fault of the Synapse folks). It defaults to false, which seems to imply that nothing is retained at all. retention_policy.enabled may have been a better name.

[brndnmtthws]

I'm a little unclear as to where your comment means you want me to update the PR or not. If there is something specific you want me to change, please let me know.