This fixes a regression that was introduced in !551.

to have all the code related to that in one place,
so it is easier to maintain and to spot bugs.

This also checks that only blogs without an existing sharing session
are shared and initialized again.
It extends an existing test to catch the missing check.

This removes some debugging information from the previous commit
to not leak private information via the sharing sessions.

Fixes #979

Also removed "personal blog" wording that doesn't apply to RSS blogs.

fix

This adds a field to the post headers and some more tests.

Attention: This factors out a DnsModule to be able to make actual
non-Tor DNS lookups for testing.

This now also handles the case where an RSS blog is deleted via the blog
deletion option and not the feed management.

This also adds a confirmation dialog to the removal process.

A fake LocalAuthor is created for this new blog and stored in the feed's metadata.