When serving Frigate at a subpath, the paths that show in the URL bar
and that wind up in your browser history are anchored at the web root.
I.e. you go to `https://example.com/frigate/`, it changes to
`https://example.com/`, and clicking around works as expected, but the
`frigate/` prefix is gone.
It's confusing if you don't know that the URL's are entirely virtual.
Also, your browser history is useless, since the URL's point to e.g.
`https://example.com/#kitchen`, but visiting that URL will not hit
`/frigate/` at all.
Most of the work is already done. Nginx injects javascript to set
`window.baseURL` based on the X-Ingress-Path header. This change passes
that to BrowserRouter, so that it'll be part of the URL's it shows.
Fixes#4526
