Added SSL Validation Override and CA Selection

- Added fields to login page to a) disable ssl validation or b) select
  a custom Certificate Authority certificate to use with the server.

- Changed visibility of widgets on login page from INVISIBLE to GONE so
  they don't take up space while hidden (since this was causing weird
  spacing issues with the new fields).

- Added state to settings to store ssl validation choice or certificate
  data.

- Added fields to various HTTP methods to disable ssl validation or set
  valid certificate authority if either setting is enabled.

app/src/main/java/com/github/gotify/messages/MessagesActivity.java

@@ -107,7 +107,7 @@ public class MessagesActivity extends AppCompatActivity
         Log.i("Entering " + getClass().getSimpleName());
         settings = new Settings(this);
 
-        client = ClientFactory.clientToken(settings.url(), settings.token());
+        client = ClientFactory.clientToken(settings.url(), settings.validateSSL(), settings.cert(), settings.token());
         appsHolder = new ApplicationHolder(this, client);
         appsHolder.onUpdate(() -> onUpdateApps(appsHolder.get()));
         appsHolder.request();
@@ -401,7 +401,7 @@ public class MessagesActivity extends AppCompatActivity
         @Override
         protected Void doInBackground(Void... ignore) {
             TokenApi api =
-                    new TokenApi(ClientFactory.clientToken(settings.url(), settings.token()));
+                    new TokenApi(ClientFactory.clientToken(settings.url(), settings.validateSSL(), settings.cert(), settings.token()));
             stopService(new Intent(MessagesActivity.this, WebSocketService.class));
             try {
                 List<Client> clients = api.getClients();