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/init/InitializationActivity.java

@@ -116,7 +116,7 @@ public class InitializationActivity extends AppCompatActivity {
     private void requestVersion(
             final Callback.SuccessCallback<VersionInfo> callback,
             final Callback.ErrorCallback errorCallback) {
-        VersionApi versionApi = ClientFactory.versionApi(settings.url());
+        VersionApi versionApi = ClientFactory.versionApi(settings.url(), settings.validateSSL(), settings.cert());
         Api.withLogging(versionApi::getVersionAsync)
                 .handleInUIThread(this, callback, errorCallback);
     }