refs https://github.com/TryGhost/Ghost/issues/9623
- add `oembed-parser` module for checking provider availability for a url and fetching data from the provider
- require it in the `overrides.js` file before the general Promise override so that the `promise-wrt` sub-dependency doesn't attempt to extend the Bluebird promise implementation
- add `/oembed` authenticated endpoint
- takes `?url=` query parameter to match against known providers
- adds safeguard against oembed-parser's providers list not recognising http+https and www+non-www
- responds with `ValidationError` if no provider is found
- responds with oembed response from matched provider's oembed endpoint if match is found
refs #9178
- move express apps to one place (called `web`)
- requires https://github.com/TryGhost/Ghost-Admin/pull/923
- any further improvements are not part of this PR
- this PR just moves the files and ensures the paths are up-to-date
