Edit and delete comments

Ok, this should be the right (and final) one..i hope
I rebased @gyst branch with the updated master and i didn't touch translations (i'll made a separate pullrequest when this is merged).

The features of this pullrequest are:
- add possibility to edit comments
- created a new permission to handle comments deletion

@gyst @tisto please check if it's all ok for you.

P.S.: there is a test (functional_test_comments.txt) that doesn't pass also on the original master in my local environment.