Fix #1592 and #1555

Allows users to search CodeMirror content with browser:
https://github.com/mybb/mybb/issues/1592

Ensures that the CodeMirror is always max. 100% width:
https://github.com/mybb/mybb/issues/1555

Also fixed tabbing issue (2 spaces were inserted due to outdated
CodeMirror option usage) and a small typo in comment.