Summary:
This refactors the flipper-server release script in such a way that it works the same as the normal release script, which solves two problems:
1) the official release script modifies versioned files, as it touches the package.json
2) it was slightly confusing that `flipper-server/static` was filled for release builds only, but not used in dev builds
3) running test:npx without running a release build before it, would fail with hard to comprehend errors. this has been solved now by removing that script and make it an arg of `build:flipper-server`
Also some further minor changes to prepare running a corresponding build / release job from SandCastle (later in this stack)
Reviewed By: nikoant
Differential Revision: D33297214
fbshipit-source-id: f6299aa982c3e59d1cc6479a93c56cbe4b57f85c
cbda298b9d