Make org logos smaller on mid-sized screens

Reduce the size of large org logos on mid-sized (tablet) screens. This
will let us fit more in on org homepages.
Use one set of featured link markup

Use the same set of markup for featured links regardless of if they want
to be used in either a news or service priority homepage.

- Update govuk_frontend_toolkit for grid_layout mixins
- Move all organisation header styles together so you can easily see
  what styles are effecting the page header, call it headings-block
  rather than block-1 so it is more obvious as to what it contains
- Wrap logo objects (main logo and parent orgs) in a single div so they
  can be floated together
- Remove old featured service and guidance links and move link to all
  service and guidance link into the header with the featured links
- Add class to the whole of the heading block to signify if it is a
  service priority page
Merge pull request #1826 from alphagov/rejig-org-pages

Rejig Organisation Pages
Use form_tag to add missing CSRF token.
Merge pull request #1833 from alphagov/csrf-on-export

Use form_tag to add missing CSRF token.
Better CSRF handling

Instead of nullifying the session (which is the default Rails behaviour), raise
an exception when an unverified request is received.