Email subject matching

Added two options:

* Remove leading occurrences of Re: and Fwd: from email subject line before using it as issue summary.
* If no issue id can be found, search for an issue with the exact same summary and append as a note there (where issue status is above read-only threshold).

Both are off by default (no changes in behavior) but can be turned on on the config page.

Use cases:
* You send a message to someone with a cc to the issue tracker email address. This creates an issue in the tracker. Any replies to the issue tracker, or to you and then forwarded to the tracker, will be added as notes.
* You send a message to another service provider and receive an automated response with a very different type of issue id. You forward it to your issue tracker (it will create a new issue), and can now forward all follow-up messages to your own issue tracker: they'll be added as notes, so that you can track the same issue in your system.
* Occassionally, spam messages get through your filters and are turned into issues in your tracker. Any further messages with the same subject line will end up as notes on the first issue, instead of flooding your tracker.