Twig font-locking

For some reason the keywords were being set to
`web-mode-uel-font-lock-keywords` when encountering `{{` tags.  As far
as I can tell, UEL does not use `{{` at all.  This was causing
`}` (used by twig in array definitions) to be erroneously highlighted.

Example:
```html
        <a href="{{ path('frontend_members_view', { 'username': username }) }}">
```

The first `}` after `username` would be highlighted.

The `web-mode-django-font-lock-keywords` were not completely
appropriate either.  This new set of keywords is a better fit for twig
and provides a place for further customization.