Refactoring: rename "updates" to "posts" throughout

We talked about renaming "updates" to "posts", to avoid confusion and just have smoother terminology.  This PR does that. Individual commits are in chunks of meaningful work, though not all tests pass at each commit. However, I'm pretty certain it's all good at the end. I've been through the web UI and tested the following:

- sign in
- post
- edit post
- delete post
- view list of posts at /posts
- view posts on /members/test1
- view RSS feed at /posts.rss (using curl)
- view RSS feed at /members/test1.rss (using curl)

You'll see there are also a few tests for RSS feeds that I included as part of this change, since they were needed and now seemed like a good time to do it.