The introduction of the new spinners brought along with it the usage of

a new LessCSS option, --rewrite-urls=all. The idea of this was to

allow consumers of a stylesheet have any URLs within it rewritten, so

that a variable that referenced a path relative to the LessCSS file

would be updated to point to the same file in the included stylesheet,

no matter where that stylesheet was located.
This worked fine for simple cases, but it broke down if all consumers of

the stylesheet didn't also use this option (which could break other

third-party stylesheets) and it triggered a LessCSS bug where URLs

passed by a consuming spreadsheet into a mixin that used detailed

rulesets would have their URLs rewritten to be relative to the file tha

defined the detached ruleset, breaking the URL.
We therefore can't use this option. This change turns it off, reverts

some updated URLs.
Since the original need was based on the spinner, the spinner now uses a

new approach. Instead of a path relative to the current file, it now

uses a path relative to the include path: djblets/images/....
This normally wouldn't work for image references, as it's intended for

@import statements, but data-uri() searches the include path, so we

can get away with it.

Tested building a package successfully, to make sure all media compiled.
Tested with Review Board, for standalone spinners, spinners on buttons

(which needed the previous URL-rewriting behavior), and datagrid icons

(as that was impacted by the URL rewriting).