Get EmailSignup to quack more like ActiveRecord

Mainly to simplify the controller and make it more conventionally
rails.
Rework tests ahead of adding functionality
Remove unused method
Make class methods instance methods

I couldn't see any reason why these methods in particular were class
methods.
Rename constant for clarity
Don't pass *args when only one arg is ever passed
Add method to verify filter option keys
Feed URL verification for EmailSignupDescription

Allows the passed in feed URL to be verified as a valid gov.uk atom
feed.
Don't symbolize keys unnecessarily
Rename EmailSignupDescription to FeedUrlValidator

Closer to what it now does.
Correct test name change
Tidy up tests to ensure valid feed urls

The protocols were invalid.
Rework EmailSignup to behave more AR-like
Make FeedUrlValidator more robust

Only try and parse the provided feed_url if the url is recognised as
a whitehall url.
Wire in feed validation to EmailSignup
Kill unused method
Add helper for generating atom feed urls

Now that we are verifying feed urls, we need the generated atom feed
urls to match the exact host for the app, i.e. Whitehall.public_host.
This won't be the case in development unless we explicitly set the
host when generating the url.