Adds welcome message on registration Welcome message is only send out if podmin account is defined and welcome message is enabled in the config.

This also enables the podmin account to send messages
to any local user.

closes #6128

closes #6118

closes #6117

closes #6111

That is, if it's the first element in the post

closes #6110

This fixes CVE-2015-3226, CVE-2015-3227, CVE-2015-1840 and CVE-2015-3225

regression from #6005

closes #6105

closes #6091

by default both are disabled now, you can enable them in the
diaspora.yml if you need them.

closes #6090

this is not included in 0.5.1.0.

[ci skip]

closes #6085

[ci skip]

closes #6082

closes #6083

refactoring:
- remove unused return-values (were used for caching, which was removed again)
- remove transaction (doesn't help here, added in 26151263)

closes #6068

closes #6060

closes #6079

closes #6069

works with rubymine

closes #6065