Allow linking to pre-filled email signup page

This has nicer URLs of the form ?organisation (rather than the nested
form Rails params), and is now enabled and dynamically updated on
announcements and publications filter pages.
Merge pull request #469 from alphagov/email-signups-from-filter-pages

Allow linking to pre-filled email signup page

https://www.pivotaltracker.com/story/show/48180175