[fix] url bar autocomplete (opensearch suggestions)

Since #2593 is merged the OpenSearch-Format is buggy.  The loop in [1] will
change raw_text_query object and this will change also the value of
`raw_text_query.query` on every `raw_text_query.changeQuery(result)`.

This patch fixes this issue by storing the initial query value in `sug_prefix`.

[1] https://github.com/searx/searx/blob/ac0fdc3b9670168de8061ed0e656dd66a567d741/searx/webapp.py#L804-L806

OpenSearch-Format::

    [ "<query>",
      [ "<term 1>", "<term 2>", ... "<term n>" ],
      [ "<content 1>", "<content 2>", ..., "<content n>" ],
      [ "<url 1>", "<url 2>", ..., "<url n>" ]
    ]

- https://www.google.com/support/enterprise/static/gsa/docs/admin/current/gsa_doc_set/xml_reference/query_suggestion.html#1080002
- https://developer.mozilla.org/en-US/docs/Archive/Add-ons/Supporting_search_suggestions_in_search_plugins#implementing_search_suggestion_support_on_the_server

Legacy-Format::

    [ "<term 1>", "<term 2>", ..., "<term n>" ]

- https://www.google.com/support/enterprise/static/gsa/docs/admin/current/gsa_doc_set/xml_reference/query_suggestion.html#1081079



Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>