Add a cleanup function

Problem/Motivation

The module currently puts no limit on the number of log entries in the system. This may lead to a very large logging table. I have a project where the table for this module is a little over 6GB.

Steps to reproduce

* Enable the module
* Generate a lot of entity operations (sorry, not quite sure how one would do this)

Proposed resolution

Implement a configuration option and a hook_cron implementation to purge log entries older than a certain age. Offer the following options:

• Never purge (default for existing installations, in order to not introduce new behaviour)
• 1 year
• 180 days
• 90 days (default for new installations)
• 30 days
• 2 weeks
• 1 week
• 1 day

Remaining tasks

• Create MR
• Review
• Merge

User interface changes

There will be a new configuration option at /admin/config/system/entity_logger

API changes

None.

Data model changes

None.