This upgrades our version of CodeMirror from 5.48.2 to 5.62. There's a

number of fixes and additions for syntax highlighting, some edge cases

fixes for browser compatibility, and several other improvements.
We're now including the mark-selection addon, which we're using to fix

the styling of code literals (which would previously hide the

selection). The selected portion will now use the default selection

color.

Tested standard CodeMirror behavior in the comment dialog and review

request fields. Didn't note any regressions.
Went through the release notes for CodeMirror to check what could

regress us, but didn't see anything that was a concern.
Tested selecting text and ensuring that code literals would properly

appear selected.