Ensure the page can be pinch to zoomed

[NickColley]

We have intentionally tried to disable pinch to zoom in the past based around the sticky navigation, but this is an accessibility issue. This pull request updates this decision to pinch to zooming, which for the most part is how it's working at the moment anyway since a lot of devices ignore blocking pinch to zoom.

I've tested in latest versions of iOS and Android.

• iOS v10 onwards ignores disabling pinch to zoom
• Android does not ignore disabling pinch to zoom, although the user can override this.

I have not been able to introduce any negative impact for allowing pinch to zoom, the interface scales proportionally and does not obstruct anything. So given that iOS already ignores this I think we should allow this for all users.

This is also a WCAG accessibility requirement.

Fixes https://github.com/alphagov/tech-docs-gem/issues/110

[NickColley]

I tested this with browserstack, which allows pinch to zoom gestures via a trackpad.

@36degrees @hannalaakso I would appreciate you double checking my findings if you have the time.

[hannalaakso]

Results of my testing in the lab: Safari on iOS and Samsung Internet on Android looks good. Chrome on iOS and Firefox on Android don't seen to work.

[NickColley]

@hannalaakso can you give more detail on 1. what URL you were testing 2. what happened to make it seem broken. This change uses the same meta tag as the Design System so I'm confused why pinch to zoom wouldnt work with this change.

[hannalaakso]

Sorry for the terseness, I was rushing off into a meeting 😞

I tested the pinch to zoom on https://tdt-documentation.london.cloudapps.digital.

I've since managed to install Chrome on the new iOS devices in the lab and pinch to zoom works on those so the issue on iPhone 5s could be a hardware issue on older iPhones or something to do with iOS 12 (although @nickcolley's findings above suggest this shouldn't be an issue > iOS 10 - I'll try to find another iOS device to test on if we want to dig into this further).

✅ Safari on iPhone 5s 12.1 ❌ Chrome on iPhone 5s 12.1 ✅ Safari on iPhone 8 13.2 ✅ Chrome on iPhone 8 13.2 ✅ Chrome on iPad 3 13.2 ✅ Safari on iPad 3 13.2 ✅ Samsung Internet 10.1 on Galaxy S8 ❌ Firefox 68 on on Galaxy S8

* Pinch to zoom works on https://design-system.service.gov.uk/

[NickColley]

@hannalaakso I think those findings confirm what I found, since this pull request has not been deployed yet I would expect some devices to not allow pinch to zoom right now.

[NickColley]

OK I'll add a changelog entry and we can get this merged, thanks for the extensive testing Hanna