For years now, we've used Font Awesome's spinner capabilities whenever

we wanted to show a spinner. This was an easy approach and had the

benefits of scalable graphics and reliable alpha-transparency (something

we didn't necessarily have on all supported browsers years back), but it

suffered from wobbly animations due to pixel precision and sizing issues

in Font Awesome, and from a lot of required setup to just display a

spinner.
This change introduces a replacement spinner, used by simply adding a

<div class="djblets-o-spinner"></div> to a page. This makes use of

SVGs, embedded within the stylesheet using data-URIs (which gives us

instant rendering without more server round-trips), and gives us all the

benefits of the old Font Awesome spinner without the downsides.
We have two variations on the spinner: A dark version, and a light

version. These are available for the different backgrounds that the

spinners might be placed on. Dark spinners are the default, but

stylesheets can call a new #djblets-ns-ui.spinners.set-theme(light) to

enable the light spinner for a given selector.
The new spinners are also sized appropriately for whatever line of text

they're currerntly on. That means that they won't cause any jumps in

height for parent elements due to discrepencies between text and spinner

sizes.
In order for the new theme variants on spinners to work, we need to

enable a new option for LessCSS, --rewrite-urls=all. This enables some

useful relative URL normalization when importing stylesheets. If

Stylesheet A has a relative URL, and Stylesheet B imports Stylesheet A,

the relative URLs inside it will be normalized correctly so that

Stylesheet B can reference the correct files. This will require the same

option to be added to consuming applications.

Tested this spinner in several places where we currently use the older

spinners.
Tested this in some upcoming changes, which place spinners in Config Forms

list item buttons. Saw that the size stayed correct.
Also tested with theme variant overrides from other stylesheets.