Update blog backend to match current usage

The blog backend supports some features we're not using:

• Multiple blogs per author
• Titles for blogs and posts
• Descriptions for blogs

On the other hand, the backend doesn't provide any support for distinguishing between imported RSS posts and native Briar posts.

We have to balance two concerns: keeping the implementation flexible so that we can add new features, and keeping the implementation simple so it's easier to maintain and audit. Forward compatibility isn't a major concern: we're going to be breaking compatibility in many other ways over the coming months, so keeping the blog protocol forward compatible while everything else breaks won't achieve anything.