Announce discussions RFC

Here are some changes I'm thinking about regarding announcements, and I'm hoping for some feedback.

1. Rename "Announce" to "Pin" and indicate pinned posts with an icon. The reasoning is I'd really like to eventually use what we now call announcements for what we now call messages. Pinning also has a natural icon which will help clean up the display and allows us to move towards discussions fitting on a single line while in lists.

2. Allow discussions to be pinned normally or as an alert. If you pin a message as an alert then you choose one of the three alert backgrounds and the pinned discussions's body will show up as a summary in the list (breaking after a certain number of characters). In table view the discussion will span all columns. I think this would actually serve the purposes that announcements currently serves better for most cases.

3. Simplify pinning to no longer use its own dialog. The pin options will show up in their own section in the discussion dropdown. Discussions will normally be pinned just within their own category. If you want to pin a discussion globally then you must pin it from the global recent discussions list. This means that message can either be pinned globally or within a category, but not both.

4. Discussions that are not pinned to the category you are viewing or discussions that have been dismissed will display as normal discussions. You won't even see a pin icon by them. This is for UX simplicity and design. Knowing that you only see pins up top is a good UX to start with. Furthermore, seeing just the pin icons up top keeps the horizontal alignment clean.