Domain-inherited purge limits and various enhancements

**Domain-inherited purge limits**
This pull request contains code to enable postfixadmin users to set purge limits on their domains, where mailboxes by default automatically inherit those values if the user doesn't specify them manually. This has the advantage of not having to change limits for every single mailbox if the limits change at any time.
To further support central administration, a new config setting ```purge_visible``` allows the settings tab in the UI to be hidden, such that only the mailserver administrator can manage limits.

**Refactored purge script**
The purge script now initializes Roundcube class to access logs and database via framework-provided methods. Also the maildir path is fetched from the database instead of manually constructing it, which greatly enhances adaptability to non-standard server configurations.