Change about:preferences style (MR 10)

- Remove policy container as it is misleading and not required to access policy
  preferences
  (see https://gitlab.com/postmarketOS/mobile-config-firefox/-/merge_requests/10#note_581408851
  for more details);
- Move search bar to the left;
- Hide Firefox Mobile ad in #sync;
- Hide Sync illustration on small screens.

src/userContent/preferences.css

@@ -0,0 +1,23 @@
+/* Copyright 2021 Anri Dellal
+   SPDX-License-Identifier: GPL-3.0-or-later */
+
+@-moz-document regexp("about:preferences.*") {
+    .sticky-container {
+        display: flex;
+    }
+
+    #policies-container {
+        display: none;
+    }
+
+    /* #sync page */
+    .fxaMobilePromo {
+        display: none;
+    }
+    
+    @media (max-width: 700px) {
+        .fxaSyncIllustration {
+            display: none;
+        }
+    }
+}