rhbz1062508 - disable spellcheck in codemirror and add key shortcut to switch between editor for current row

There is no remedy for spellcheck in codemirror. This is a workaround of https://bugzilla.redhat.com/show_bug.cgi?id=1062508

This adds a key shortcut ctrl+alt+h (and attention key h) to switch between codemirror and plain text for current row. It also changes default editor to plain text if user don't have a persisted option.

The spellcheck hack along with the option have been removed. Option description updated to warn users who choose to use codemirror.

- It will help people relying on browser spell checker to work as well as having syntax highlight available at fingertips. https://bugzilla.redhat.com/show_bug.cgi?id=1062508
-  source panel use codemirror runmode (like what's in TM) so users can right click with copy & paste option available. https://bugzilla.redhat.com/show_bug.cgi?id=1024130