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/service/WebSocketService.java

@@ -42,7 +42,7 @@ public class WebSocketService extends Service {
         super.onCreate();
         settings = new Settings(this);
         missingMessageUtil =
-                new MissedMessageUtil(ClientFactory.clientToken(settings.url(), settings.token()));
+                new MissedMessageUtil(ClientFactory.clientToken(settings.url(), settings.validateSSL(), settings.cert(), settings.token()));
         Log.i("Create " + getClass().getSimpleName());
     }
 
@@ -79,7 +79,7 @@ public class WebSocketService extends Service {
         }
 
         connection =
-                new WebSocketConnection(settings.url(), settings.token())
+                new WebSocketConnection(settings.url(), settings.validateSSL(), settings.cert(), settings.token())
                         .onOpen(this::onOpen)
                         .onClose(() -> foreground(getString(R.string.websocket_closed)))
                         .onBadRequest(this::onBadRequest)