Maintenance feature to remove old users

Add Sidetiq webview to the Sidekiq monitoring panel

Add rake task maintenance:queue_users_for_removal
This basically just triggers an immediate run of the normal maintenance remove old users functionality that is normally (if enabled) scheduled to run once a day via sidetiq

Add extra safety when checking for user removal due to inactivity.
Now also user.last_seen will also be checked to make sure a user will not be removed in the event that the Devise rememember me login functionality has stopped the users remove_after timestamp from being removed.

Add initializer for maintenance job.
Add warning about mail being disabled if remove_old_users maintenance is enabled.
Merge pull request #5288 from jaywink/remove-old-users

Maintenance feature to remove old users, scheduled in settings