Also fixed a couple of bugs.

Also made some listeners volatile.

Fixes #539, #564 and #568.

This also adds a setting allowing people to turn blog post notifications off.

Closes #488

Closes #415

This commit addes a combined blog feed that shows all posts of all
subscribed blogs in the order the blog posts have been received.

For now, this commit also hides other blog functionality like adding
additional blogs and browsing individual blogs.

Closes #417