Monkeypatch: in CodeMirror, revert less.js to a working version

For #6057, this hotfixes the CodeMirror submodule to point to a temporary branch (also named post-sprint-34-hotfix) that reverts the LESS mode to marijnh/CodeMirror@390a47e. A bug was introduced in marijnh/CodeMirror@6fd2965 that was later fixed (after our most recent upstream merge). Rather than try to take the latest upstream or monkeypatch in the bugfixes, we thought it would be safer to just revert marijnh/CodeMirror@6fd2965 since it was a rather large change to the LESS mode and we probably don't need the fixes in that commit right now.