Allow the author of a commit to submit the commit, independent of Approver role

Gerrit allows both the original author as well as the approver to stage/submit a commit. However, the informal rule in Qt Creator right now is that the original author will merge a commit: He know's best whether there are non-obvious dependencies to take into account, or how to resole a conflict in case it happens. I think this rule makes sense also as soon as we have external contributors.

Codereview right now only allows people in the "Qt Creator" or "Approver" role to submit a commit. This should be changed so that every registered user, or the original committer should have the right too.