Replace the hardcoded set of available icons in `svg-icon.js` with a
`registerIcons` function which is called by the application at startup
to register the icons that should be available for use.

This will enable us to re-use the `SvgIcon` component in the "annotator"
part of the client or other applications (as part of our UI component
library) which use a different set of icons.

The lms application currently has a different solution for this in its
`SvgIcon` implementation which requires the icon markup to be passed
each time the component is used. On reflection, I think that passing
just the name at the point of use is going to be more convenient in app
code and when writing tests.  It also avoids the need to validate the
markup is from a trusted source on each use.