Upgrade to CodeMirror 3.23.

This release fixes some issues with Markdown and some issues with
editing JSON payloads (needed for the upcoming API token policy editor).

This is not the latest CodeMirror, but we had issues with the 4.x series
(4.2, specifically) last time we tried it. There's a 3.24 release, but
it's not on the online minifier yet.

The build of CodeMirror is also updated to support JSON/JavaScript
modes, and to support optional linting and brace matching.

Reviewed at https://reviews.reviewboard.org/r/6156/
Add RB.Config.ResourceListItem for interfacing resources with configforms.

This moves over RB.Config.ResourceListItem from RBCommons. This is a
subclass of Djblets.Config.ListItem, and handles the work of interfacing
a ListItem with a resource object. It will be used for the upcoming API
token UI.

Reviewed at https://reviews.reviewboard.org/r/6150/
Fix setting the height of the review request box for long fields.

The fixes to set the proper widths of the fields broke the heights.
It removed the defer() call and replaced it with a throttle(). The
defer() is important to the calculations, though, since we schedule a
layout resize after closing CodeMirror fields but before all layout has
settled.

This restores the original _scheduleResizeLayout() function and adds the
throttle there.

Reviewed at https://reviews.reviewboard.org/r/6154/
Merge branch 'release-2.0.x'
Add documentation on API tokens.

This adds documentation on authenticating with API tokens and writing
custom API policies.

The authentication docs now cover how to log in with either API tokens
or a username/password. Some of the existing docs here are also fleshed
out to create better examples.

The new API token policy docs cover the structure of policies, the
various sections of the policy, how they affect access, and some tips.
This will complement the UI for editing tokens, which is currently in
development.

All API resource pages now also contain the policy IDs for use in the
policies.

Reviewed at https://reviews.reviewboard.org/r/6155/
Fix wrapping of file attachments.

At some point, we added another div around file attachments. When this
happened, the float rule wasn't moved to apply to the proper element.

Testing done:
Attached a bunch of files to a review request and saw that they were listed
inline instead of in one big vertical column.

Reviewed at https://reviews.reviewboard.org/r/6158/