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;
     }

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)