Spinner image doesn't render well at small size

[lyzadanger]

The Spinner component and underlying spinner pattern here are adapted from spinner use on the LMS project. In that project, the spinner is always rendered at about 2em square, i.e., large-ish.

We also want to replace uses of the old, CSS-styled spinner in the client with the new Spinner component here. However, in many cases in the client, the spinner is rendered smaller, 1em square. This works with the visual design of the client spinner, but does not work so well with the image used by the new Spinner component.

The Spinner component here uses the "tail spin" loader SVG image, which, at time of writing is the second from the left in the bottom row of these examples. That renders fine at larger sizes, but the line weight is too thin for it to work at the dimensions that the client uses. 1em in the client averages about 13px. It becomes nearly invisible at that size.

We have a couple of options:

• Find a single loader image that looks OK at larger and smaller sizes, or
• Leave the current spinner loader image alone, but add an additional loader image that can be used, either automatically when size="small" or as a manually-set variant or somesuch

It's a pity, because I think the visual styling of the current (CSS-based) client spinner is nice, but I lack the skills to be able to animated-SVG-ize such a thing.

[lyzadanger]

Fixed by #154