Add PKCE support to remoteStorage OAuth

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

app/models/rs_authorization_code.rb

@@ -0,0 +1,37 @@
+class RsAuthorizationCode < ApplicationRecord
+  belongs_to :user
+
+  CODE_TTL = 5.minutes
+
+  serialize :permissions, coder: YAML
+
+  attr_accessor :code
+
+  before_validation :generate_code
+
+  scope :active, -> { where(expires_at: Time.current..) }
+
+  def self.find_by_code(raw_code)
+    return nil if raw_code.blank?
+
+    find_by(code_digest: Digest::SHA256.hexdigest(raw_code))
+  end
+
+  def expired?
+    expires_at <= Time.current
+  end
+
+  def consume!
+    destroy!
+  end
+
+  private
+
+    def generate_code
+      return if code_digest.present?
+
+      self.code = SecureRandom.urlsafe_base64(32)
+      self.code_digest = Digest::SHA256.hexdigest(code)
+      self.expires_at ||= CODE_TTL.from_now
+    end
+end