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.

This was preventing introduction messages from getting ACKed.
The introduction tests were modified to check for this.

Evidently this way of nagging myself to write tests doesn't work.

and include in canBeOpened calculation for private groups and sharables.

Fixes #816

after the invitation.

Closes #876