Show registration unavailable page when signup is disabled

Instead of showing the signup form and returning an internal error on
submit, the SignUpPage now queries signUpEnabled upfront and displays a
friendly message explaining that registration is not available, with a
link back to login.

Adds a signUpEnabled GraphQL query field on the connect/v1 API and
handles ErrSignupDisabled as a FORBIDDEN error in the SignUp resolver.

Signed-off-by: Sacha Al Himdani <sacha@getprobo.com>

pkg/server/api/connect/v1/schema.graphql

@@ -55,6 +55,9 @@ type Query {
   oidcProviders: [OIDCProviderInfo!]!
     @goField(forceResolver: true)
     @session(required: OPTIONAL)
+  signUpEnabled: Boolean!
+    @goField(forceResolver: true)
+    @session(required: NONE)
 }
 
 type Mutation {