Show an error if a server has signups closed

closes #377

mastodon/src/main/java/org/joinmastodon/android/fragments/onboarding/InstanceCatalogFragment.java

@@ -296,6 +296,14 @@ public class InstanceCatalogFragment extends BaseRecyclerFragment<CatalogInstanc
 	private void proceedWithAuthOrSignup(Instance instance){
 		getActivity().getSystemService(InputMethodManager.class).hideSoftInputFromWindow(contentView.getWindowToken(), 0);
 		if(isSignup){
+			if(!instance.registrations){
+				new M3AlertDialogBuilder(getActivity())
+						.setTitle(R.string.error)
+						.setMessage(R.string.instance_signup_closed)
+						.setPositiveButton(R.string.ok, null)
+						.show();
+				return;
+			}
 			Bundle args=new Bundle();
 			args.putParcelable("instance", Parcels.wrap(instance));
 			Nav.go(getActivity(), InstanceRulesFragment.class, args);

mastodon/src/main/res/values/strings.xml

@@ -388,4 +388,5 @@
 	<string name="privacy_policy_subtitle">Although the Mastodon app does not collect any data, the server you sign up through may have a different policy. Take a minute to review and agree to the Mastodon app privacy policy and your server\'s privacy policy.</string>
 	<string name="i_agree">I Agree</string>
 	<string name="empty_list">This list is empty</string>
+	<string name="instance_signup_closed">This server does not accept new registrations.</string>
 </resources>