Update 'Buttons' in line with style guide

- 'like' rather than 'such as'
- 'should be avoided' -> 'you should avoid'
- Link to user research pages
Rewrite pages about frontend repositories

Sass repositories:

- More clearer words
- Typo 'forth' -> 'fourth'
- Less Title Case

Shared asset libraries:

I think this page was originally created when the GOV.UK template didn't
exist, so talk about that and how we use it.
Rewrite page on 'Grids'

- Remove unnecessary 'Guidance' heading
- Use definition list for the mixin arguments
- Correctly indent code blocks
- Reduce jargon
Update page on 'Creating accessible PDFs'

- Remove unnecessary first heading
- 'san serif' -> 'sans serif'
- Update URLs that redirect
- Remove trailing semicolons from bulleted lists
Update page on header/footer

- Capitals
- 'Your tax account' is something that we *specifically* flag on the content
  designers style guide that shouldn't be title case, so it's kind of embarrassing
  that we were doing exactly that here
- End users are just users
- Use HTTPS when connecting to www.ietf.org
Update page on forms

- 'radios' -> 'radio buttons'
- Change references to 'we' to 'you' like the rest of the manual
- Improve validation paragraph
- Make links relative rather than to www.gov.uk
Update user centred design resources

- Use HTTPS when connecting to Wikipedia
- I'm not convinced by the CAPTCHA security argument, so use 'may'
- Better link titles
- Remove links from the footer where they're linked to in the page
- Fewer capitals
Fix JavaScript in user centred design resources

- The JS in the overall forms page isn't actually targeting anything
- The JS in the example form page doesn't work because it's run before
  jQuery is loaded
Remove FAQ-like thing

- style guide violation
- doesn't add anything to the page structure
Dash style
Merge pull request #342 from alphagov/user-centred-design-resources-update

Update "User centred design" resources