Edit MR comment instead of creating a new one

Messages to MRs from the nightlies will be appended to the last comment from the lhcbsoft user. If there is no such comment, a new one will be made. Datetime tags are added to identify the individual messages.

The comment will look something like this(check the source of the description).

• [2017-04-23 00:01] Automatic merge failed in lhcb-gaudi-head#1451
• [2017-04-24 00:02] Validation started with lhcb-gaudi-head#1451

GitLab does not send notifications on edits, which will significantly reduce spam. Thus the notifications for HEAD are switched on.

Additionally, a different message is posted when the automatic merge fails (links to the checkout log).