This commit refactors the code for sharing forums,
so it can be used for sharing blogs as well.
It does not yet include code for responding to blog invitations.
This prevents trying to access the same group metadata in different groups.
Also, the conversation does not need to be reloaded once introduction messages arrive.
Closes#535
When a contact is added, her personal blog will also be added automatically.
Also, when a new identity is added, a personal blog for that identity is created.
Part of #436
Removes teaser and makes body mandatory.
It also adds support for deleting blogs and
introduces a getAuthorStatus() method to the IdentityManager
that takes a running transaction.
