Remove Rocketchat

• also from documentation

changed milestone to %0.8.0

added 1 deleted label

It was removed from stackspin.net, so we can remove it from this repo too

added Doing label and removed 1 deleted label

assigned to @varac

created merge request !656 (merged) to address this issue

mentioned in merge request !656 (merged)

mentioned in commit a1e991ee

marked the checklist item also from documentation as completed

added QA label and removed Doing label

mentioned in commit 46ab62db

mentioned in commit 4321a8cf

closed with commit 46ab62db

closed with merge request !656 (merged)