Evaluate unread emails that are older than 15 minutes for testing purposes

[andreaslampe]

Setting up regular expressions is complicated. To test them, it's necessary to run tests against older emails. Therefore, it's necessary to have a configuration entry that lets you deactivate the threshold of 15 minutes.
Please make sure a warning is displayed during startup to avoid having this enabled in production mode.

[andreaslampe]

And of course, please don't forget to update the configuration.