Vue Router generates URLs for which there are no static files on the
filesystem (like `/jobs/{job ID}`). To make this work, the webapp's
`index.html` has to be served for such requests. The client-side JavaScript
then figures out how things fit together, and can even render a nice 404
page if necessary.
This shouldn't happen for non-webapp URLs, though. Because of this, the
entire webapp (including the "serve `index.html` if file not found logic)
is moved to a `/app/` base URL.
`make flamenco-manager` now also builds the webapp and embeds the static
files into the binary.
`make flamenco-manager_race` does NOT rebuild the static web files, to
help speed up of debug cycles. Run `make webapp-static` to rebuild the
webapp itself, if necessary, or run a separate web development server with
`yarn --cwd web/app run dev --host`.
