Implements the optional OAuth 2.0 Authorization Code grant with PKCE
(RFC 7636) for remoteStorage, as described in the remoteStorage spec
(section 10.1).
- Advertise the authorization and token endpoints plus the supported
code_challenge method (S256) in WebFinger, and bump the advertised
spec version to draft-dejong-remotestorage-27
- Add RsAuthorizationCode: a short-lived (5 min), single-use authorization
code stored only as a SHA256 digest
- Add RsOauthManager services for issuing and exchanging codes, including
S256 verification
- Add a cross-origin POST /rs/oauth/token endpoint that exchanges a code
(and code_verifier) for an access token, with CORS and no CSRF token
- Consent dialog now carries response_type / code_challenge through the
form and redirects with the code in the query string for the code flow
- Remove the existing-token reuse shortcut so every authorization results
in a distinct authorization (and token)
Part of #254
