roundcube: New app for web based email client

Created by: SunilMohanAdapa

This pull request add an app for configuring Roundcube. The database backend will be sqlite3 and I believe this is sufficient because Roundcube does not fetch and index all the messages. It relies on IMAP server to do that while trying to be lightweight IMAP client. Using sqlite3 I believe should keep things simple in terms of resource necessity on FreedomBox.

It is possible to use any external IMAP server with or without SSL. I have tested with my email provider as well as with Gmail. Sending mails is not tested.