Add opt-in full-width tablet breakpoint

[rpowis]

Problem

API Platform have an unusual UX use case.

End users of third party software have to complete an "authorisation journey" on MDTP to get something called an access token, which their software can then use to call our APIs. The authorisation journey includes: a start page, sign in, all the 2SV pages, all the IV pages and a "grant authority" page.

A lot of third party software is fat client, so the authorisation journey is run in a browser launched by their software - usually IE and often at a specific page width of 650px.

MDTP pages use the standard two-thirds/one-third page split at 650px, meaning a lot of word wrapping on the left and a lot of whitespace on the right.

Solution

Add an opt-in breakpoint for services that need to be full-width at tablet viewport widths.

Notes

• It's possible to achieve this layout using the latest govuk-frontend in the form of a *-from-desktop class (added Add desktop specific grid classes alphagov/govuk-frontend#1094).
• We've named ours to be more in keeping with the existing assets-frontend .full-width class.
• Teams moving from assets-frontend to govuk-frontend will need to update their class names to adopt the new styles anyway. Having to do the same for this class too isn't creating massive overhead.