Add support for tagging patches (and/or indicating the branch they are based on)

I am not sure whether universal and arbitrary tagging support is required or good in the first place, so I'll just desribe the use case I'd like, and a proper solution can be figured out from there.

I'd like to be able to mark patches as being fixes, specifically fixes that I need to queue to drm-intel-fixes or drm-intel-next-fixes. I'd like to be able to use patchwork to communicate that these patches need more urgent review, and testing, than others, because the schedule for the patches is different. I'd like other people to flag patches as fixes, communicating to me I need to consider them for said branches.

Clearly this is distinct from the patch state.

There will also arise the need to indicate which branch a patch is based on. This information will be useful for QA. Typically we develop on top of nightly, but some patches, particularly fixes, only apply and need testing on a specific branch. In my case, the tagging described above may mean the same thing as indicating the branch. Maybe these need to be separate things. Maybe the branch indication is a more urgent feature, and could be used for starters. (Maybe we can interpret the patch subject line, and set the branch accordingly if the patch has e.g. [drm-intel-fixes PATCH].)

I currently just tag the patches as part of my email flow, and manually, semi-regularly push a view to my tags at http://people.freedesktop.org/~jani/drm-intel-fixes. The bottom line is that I'd like that to be more collaborative, more automated, and more integrated to patchwork.