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.

The toolbar subtitle shows information about how many contacts the
current blog is shared with and how many of those are online.

* Remove distinction between "shared with" and "shared by"
* Show all contacts a blog is shared with
* Show online status of contacts in sharing screen

This adds a sign() and a verify() method to the CryptoComponent
that take a mandatory label argument to ensure that signatures can't be
repurposed.

and make sure work is offloaded to an executor, so exceptions can be
caught.