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/WebSocketConnection.java

@@ -15,15 +15,9 @@ import okhttp3.WebSocket;
 import okhttp3.WebSocketListener;
 
 public class WebSocketConnection {
+    private OkHttpClient client;
     private static final JSON gson = Utils.json();
 
-    private final OkHttpClient client =
-            new OkHttpClient.Builder()
-                    .readTimeout(0, TimeUnit.MILLISECONDS)
-                    .pingInterval(1, TimeUnit.MINUTES)
-                    .connectTimeout(10, TimeUnit.SECONDS)
-                    .build();
-
     private final Handler handler = new Handler();
     private int errorCount = 0;
 
@@ -38,7 +32,15 @@ public class WebSocketConnection {
     private Runnable onReconnected;
     private boolean isClosed;
 
-    WebSocketConnection(String baseUrl, String token) {
+    WebSocketConnection(String baseUrl, boolean validateSSL, String cert, String token) {
+        OkHttpClient.Builder builder = new OkHttpClient.Builder()
+                .readTimeout(0, TimeUnit.MILLISECONDS)
+                .pingInterval(1, TimeUnit.MINUTES)
+                .connectTimeout(10, TimeUnit.SECONDS);
+        Utils.applySslSettings(builder, new Utils.SSLSettings(validateSSL, cert));
+
+        client = builder.build();
+
         this.baseUrl = baseUrl;
         this.token = token;
     }