Remove feature toggle around newsletter subscription form.

Reverts the following commits:

- "Add feature toggle around newsletter subscription form." 9d5b5c6c19baa6c625036783857e2156122644bc
- "Use background step to check for the state of the feature." a1c32333e6fd7dbf5eed240df22968f1f88e74c6
- "Run the newsletter subscription acceptance tests given the feature is enabled." 0b5a7a06322a05cfbda13f5d5562f5c3d1e5b566
- "Little trick to hide the grey line if the newsletter subscription form is hidden." 1055982460009bc2933755b30e6ad39d3fcdc6ec

@ALiaume Newsletter signup feature ready to go live.
Update newsletter subscription heading to H2
Merge branch 'email-headingd-fix'
Add skip to main link to the styleguide pages
Merge pull request #518 from moneyadviceservice/fix/skip-to-main-nav

Add skip to main link to the styleguide pages
Add GB suffix to alternate metatags
Merge branch 'bug/alternates-sufix'
Use HTTPS map image to avoid security warning
Prepare the category tree on application init
Merge branch 'warmup-category-cache-on-init'
Only warm category cache when rails server is being started
Use public website database in development env
Share sessions with public_website
Add Authentication concern
Render login/logout links in site header
Add "register" and "my account" links where appropriate