Styling for the release page plus some refactoring

[helenb]

NB this is still a work in progress as some elements require further design tweaks - particularly the census logos and the accreditation content.

What is the context of this PR?

• Tidied up commenting on scss files
• Refactored some heading styles to make them common to all pages
• Moved release styles to the common-header
• Fixed spacing with the accredited logo
• Added conditionals around page content where necessary
• Create a re-usable spacing class for streamfield and other content so we have consistent spacing between page blocks
• Adjusted spacing below paragraphs for all rich text blocks after discussion with Gareth

How to review

In your local build, check out this branch, and view a release page. Check it in all it's various different states.

Follow-up Actions

List any follow-up actions (if applicable), like needed documentation updates or additional testing.

[helenb]

This change is just to remove a wrapper class and re-indent.