Update to latest CodeMirror upstream master
- Update SHA to our CodeMirror fork's master, which is now clean with upstream
  (brackets-sprint20 branch in the adobe/CodeMirror2 fork is no longer needed)
- Fix htmlmixed mode to specify parameters for handling mustache/handlebars
  templates, as required by latest CodeMirror
- Reinstate commented-out unit tests from #2887 since the underlying CM bug
  should now be fixed