Optionally allow discourse to create new topics from email.
Refactor and clean up New-Topic via Email

With the new email_in admin configuration setting, emails to the email_in_address fetched via POP will now be processed and posted as new topics to the forum.

With the email_in_min_trust you can control the trust level the user needs to have at least to be able to post an email as a new topic.

Also contains tests for the email-in feature and minor clean ups
Add Email-In-Per-Category

 - allow the configuration of an inbox-email-address per category
 - post emails to that email into that category instead of global

 - Adds UI for configuration
 - Adds Documentation for configuration
 - Adds Tests for new feature
Add public-inbox to Email-In-Feature

 - Adds the advanced option to accept email from non-users per category email-address
 - Adds tests covering the new feature
 - Adds UI to configure this feature in the frontend
Switch to proper exception handling system for better user feedback

 - Replace implicit return code-system in Email::Receiver with proper exception system
 - Update tests to check for exceptions instead
 - Test the PollMailbox for expected failures
 - Add proper email-handling of problematic emails
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Merge pull request #2012 from ligthyear/incoming-emails

Advanced New-Topic via Email Feature