View/edit browser config in a special browser page

Add the ability to view/edit the browser's config settings via a special
browser page.

It's very simplistic for now, but a good starting point.

Future possibilities:
* Matching config types to html input types
  * colors = html colorpicker
  * options with valid_values = select/multi-select
  * plain text fallbacks where appropriate
  * multi-line text edits for long options (i.e. host-block-lists)

* Javascript option verification
  * switch from submitting changes onblur to onchange if an option passes basic
    verification, etc.
Pass around win_id so we can use it in the error message handler

The message class needs a win_id to know where to send error messages. Just
pass it through the jinja->js->qtwebbridge as it's just a simple int.
Fix problem with qutesettings scope in pages

Fix to make sure the js bridge code is only enabled when qute: pages are shown.
Previously it would only be available to the first page (and before that it
was available to all pages).
Merge branch 'master' of https://github.com/iggy/qutebrowser into iggy-master
Don't add javascript-bridge only once.

Otherwise, qute:settings will break when reloading it.
Fix lint
Merge branch 'qute-settings'