Add asset links for not included art options

For now they are the same links as we have ingame. If they are changed I recommend to change all of them.

Also dedupe the CI definitions a bit.

@Elohiem When updating the webgl stuff make sure you update the link in .gitlab-ci.yml too. Simply replace WEBGL_URL with your link.

@Pregmodder If you'd rather want links to your MEGA account feel free to change them, but since we already have them ingame I believe they are ok for now.

@svornost Since you have worked a lot on the vector art rendering, how feasible is it to move the SVGs outside src/ and offer them as an optional download, similar to the WebGL approach?

part of #2915