userChrome/browser: move nav bar to bottom (MR 19)

Based on MR 17 by plata-gl, thank you very much, and thanks to everybody
who answered the survey linked in that MR whether to make this default
or not.

I've left out padding-top since the additional 1px bar towards the top
looks like a bug, I've compared with Fennec on Android and it doesn't
have such a line. Also I've left out the inFullscreen lines from the MR,
not sure what they are for - we can still add them in a follow-up patch
if desired.

src/userChrome/browser.css

@@ -0,0 +1,9 @@
+/* Copyright 2021 plata
+ * SPDX-License-Identifier: GPL-3.0-or-later */
+
+/* Move navigation bar to bottom */
+@media (max-width: 700px) {
+    #browser {
+        -moz-box-ordinal-group: 0 !important;
+    }
+}