Add automatic pruning of old/broken data

[dsebastien]

Old data should be cleaned up from time to time.
We could use the Prunable or MassPrunable traits to implement a part of this: https://laracasts.com/series/lukes-larabits/episodes/21

• Implement
• Add documentation
• Automate for production

To perform the operation: php artisan model:prune. Add --pretend to check before performing it

Examples of things to clean up:

• Unused accounts
• Resources that are not available anymore anywhere (404 and/or not used in any community)
• ...