Implement OpenID module (#599) (#1812)

* Implement OpenID module (#599)

- Unrelated: change Riot references to Element in client API routing

Signed-off-by: Bruce MacDonald <contact@bruce-macdonald.com>

* OpenID module tweaks (#599)

- specify expiry is ms rather than vague ts
- add OpenID token lifetime to configuration
- use Go naming conventions for the path params
- store plaintext token rather than hash
- remove openid table sqllite mutex

* Add default OpenID token lifetime (#599)

* Update dendrite-config.yaml

Co-authored-by: Kegsay <kegsay@gmail.com>
Co-authored-by: Kegsay <kegan@matrix.org>

dendrite-config.yaml

@@ -360,6 +360,11 @@ user_api:
     max_open_conns: 10
     max_idle_conns: 2
     conn_max_lifetime: -1
+  # The length of time that a token issued for a relying party from 
+  # /_matrix/client/r0/user/{userId}/openid/request_token endpoint
+  # is considered to be valid in milliseconds. 
+  # The default lifetime is 3600000ms (60 minutes).
+  # openid_token_lifetime_ms: 3600000
 
 # Configuration for Opentracing.
 # See https://github.com/matrix-org/dendrite/tree/master/docs/tracing for information on