Don't autoclose strings

CodeMirror doesn't do the right thing with triple-quoted strings,
so don't let it try.
Merge pull request #5372 from minrk/no-close-strings

Don't autoclose strings, because CodeMirror's behavior makes it confusing and awkward to type triple-quoted strings (and these are very common in Python).

An issue about this was filed in CodeMirror itself: https://github.com/marijnh/CodeMirror/issues/2385