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/Settings.java

@@ -34,6 +34,8 @@ public class Settings {
     public void clear() {
         url(null);
         token(null);
+        validateSSL(true);
+        cert(null);
     }
 
     public void user(String name, boolean admin) {
@@ -57,4 +59,13 @@ public class Settings {
     public void serverVersion(String version) {
         sharedPreferences.edit().putString("version", version).apply();
     }
+
+    // Default to always validating SSL.
+    public boolean validateSSL() { return sharedPreferences.getBoolean("validateSSL", true); }
+
+    public void validateSSL(boolean validateSSL) { sharedPreferences.edit().putBoolean("validateSSL", validateSSL).apply(); }
+
+    public String cert() { return sharedPreferences.getString("cert", null); }
+
+    public void cert(String cert) { sharedPreferences.edit().putString("cert", cert).apply(); }
 }