Automatically set the redirect URI in OIDC

When we first added OIDC support, we were configuring the redirect URI
in the devise initializer, just like we did for other providers.

Thanks to the changes in the previous commit, that code is no longer in
the devise initializer, which means we can use `url_helpers` to get the
redirect URI.

This means we no longer need to define this URI in the secrets. This is
particularly useful for multitenancy; previously, we had to define the
redirect URI for every tenant because different tenants use different
domains or different subdomains.

config/secrets.yml.example

@@ -97,7 +97,6 @@ staging:
   oidc_client_id: ""
   oidc_client_secret: ""
   oidc_issuer: ""
-  oidc_redirect_uri: ""
   <<: *maps
   <<: *apis
 
@@ -160,7 +159,6 @@ preproduction:
   oidc_client_id: ""
   oidc_client_secret: ""
   oidc_issuer: ""
-  oidc_redirect_uri: ""
   <<: *maps
   <<: *apis
 
@@ -222,6 +220,5 @@ production:
   oidc_client_id: ""
   oidc_client_secret: ""
   oidc_issuer: ""
-  oidc_redirect_uri: ""
   <<: *maps
   <<: *apis