It is possible that a remote DECLINE message arrives short before the
user responds to the introduction.
This will cause a ProtocolStateException which (for now) is just caught
and a generic (existing) error message will be shown.

A fake ACTIVATE message would cause us to activate the transport keys
before the contact has received our auth message,
which would compromise forward secrecy.

When the user attempts an introduction, instead of the introduction
message input field, an explanatory text will be shown and the
introduction can not be made until the last one has been finished.

Closes #308

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

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.