Make default query submission method configurable

Sending queries through POST, while better for privacy, breaks functionality
with certain extensions (e.g. Firefox containers). Since Firefox does
not send cookies when requesting `/opensearch.xml`, users cannot easily
switch to GET on the client side unless they make a custom search
engine. This commit allows admins to modify the default method on their
side so they can set it to GET if needed.

searx/preferences.py

@@ -348,7 +348,7 @@ class Preferences(object):
                 }
             ),
             'method': EnumStringSetting(
-                'POST',
+                settings['server'].get('method', 'POST'),
                 choices=('GET', 'POST')
             ),
             'safesearch': MapSetting(