Add WAI-ARIA attributes to graph elements

This commit adds role=presentation and aria-hidden=true to all
SVG elements and figcaptions used for linelabels.

This prevents screenreaders from reading out the elements.

As noted in the comment, the WAI-ARIA spec says we may do this
with caution to improve the experience for assistive technologies.
Merge pull request #138 from alphagov/61980510-hide-graphs-from-screenreaders

Add WAI-ARIA attributes to graph elements
Minimum viable GOV.UK dashboard

- Add new stagecraft stub for a GOV.UK dashboard
- Allow the tagline to accept HTML so it can contain links to external
  pages
Add realtime usage module to GOV.UK dashboard
Merge pull request #139 from alphagov/govuk-dashboard

Minmum viable GOV.UK dashboard
Make "more info" sections accessible, using <details> tag
Merge pull request #140 from alphagov/make-more-info-accessible

Make "more info" sections accessible, using <details> tag
Add aria-labelledby attribute to module section

Allows sections to be more easily navigated by screen-reader / keyboard
users.
Merge pull request #137 from alphagov/61977424-aria-labelledby

Label module sections for better, more accessible navigation
Update callout copy
Update user stage info text to mention events
Remove extra line break
Tests, huh, what are they good for
Update copy to use measured
Merge pull request #142 from alphagov/revise_user_stage_copy

Revise user stage copy
Update govuk_template to 0.4.0

Removes the non-breaking space from the header.

Improves propositional navigation.

Crushes PNGs (with thanks to @jabley).

https://github.com/alphagov/govuk_template/compare/v0.3.7...v0.4.0
Merge pull request #145 from alphagov/update-govuk-template

Update govuk_template to 0.4.0
Add complementary role
Add main role
Add navigation role