The URLS file is no longer necessary as urls are now generated directly from the app VHOST file.
This also moves all the url generation logic to the domains plugin.
This change also moves the referenced Procfile out to a host path once on deploy vs potentially several times, which should speed up deploys a small amount and simplify reasoning about the file.
Closes#5112
The previous method was a bit more difficult for users to interact with as they needed to ensure the file was at a certain path in the built image. This change divorces the file from the built artifact, better aligning the nginx.conf.sigil handling with the rest of Dokku's monorepo support.
A side effect of this is now building proxy configurations does not require access to the built image.
Closes#5207
