This change modernizes the General Settings page a bit, focusing on the

cache backend settings. We don't have pluggable cache backends, like we

do with many other parts of the product, so the General Settings page

has traditionally managed the settings for memcached, file-based cache,

and local memory cache in a pretty hard-coded way, and this didn't work

well with browser-side validation.
The cache backend configurations are now managed by their own forms, and

registered in a central place. This doesn't give us pluggable backends,

but it's a step toward allowing that in the future without having to

revisit all of this code.
This leaves the main form without all the hard-coded mappings and

constants, leaving all knowledge of explicit cache backends to the

registration dictionary. It also lets us use the new subform support to

manage the display and validation of the correct form for the selected

cache backend.
Since this is being changed anyway, some tweaks were made to help guide

users and prevent issues. The "Local Memory" cache backend is now only

enabled for development (non-PRODUCTION) installs, rather than being

keyed off by the DEBUG setting. This prevents it from showing up if an

install has been flipped to debug mode.
The option for the memcached backend now lists "(recommended)" after it,

giving users a hint that they should probably choose that rather than

the file cache backend.
There's also a tweak to a template location, to fix the path that's

referred to by Djblets for some of the fieldset rendering.
With all this, the General Settings page should now be complete and

operational for Review Board 4.0.

Tested loading and saving all of the cache backends in the General

Settings page, verifying the results in each case.