Configure Vue to not assume it's on the web root

[growse]

Not sure if this will have much knock-on effects, but when trying to run the frontend on a non-root path, I noticed that the generated output assumes that config etc are all available on /, which isn't necessarily the case.

Setting publicPath to be "" seems to make it try and resolve dependencies relative to index.html, which feels like the right thing to do.

[linusg]

tested it locally and it breaks the map as its route is /. We might have to add another config option similar to api.baseUrl as we don't know it during build time (anymore) but have to configure the router correctly:

frontend/src/router.js

Line 9 in 4d971d5

base: process.env.BASE_URL,