Pull Material Icons assets from npm

[eritbh]

We currently use an old version of https://github.com/marella/material-icons with slight modifications to use the tb-icons class rather than the standard material-icons. It'd be nice if we could source the assets for this from npm and use our build step to copy the assets from node_modules into the extension, rather than having that in-tree.

Alternatively we may want to do something completely different to use icons from React code, I have no idea what the standard there is yet.