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

* Add a string label argument to CryptoComponent#hash()
* Convert DoubleDigest from implementing MessageDigest
  to implementing org.spongycastle.crypto.Digest
  (we need to keep DoubleDigest for FortunaGenerator)
* Convert all other uses of MessageDigest to CryptoComponent#hash()
* Remove CryptoComponent#getMessageDigest(), MessageDigest and DigestWrapper

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.

interface and a factory for that.