feat(web): optional override of iOS Safari "Request Desktop Website" setting

In iOS 13 Apple added the ability for Safari to set a "Request Desktop Website" option that then spoofs a desktop-macOS user agent string when enabled. (See also #2267.) It's enabled by default on iPads now, which caused significant breakage in our iOS app. This has been mitigated for stable with #2268, with the comment below in regard to a more complete desired implementation:

> There's one challenge here -- if a user has requested the desktop site but we use the mobile version of KMW, then there may be a disconnect in layout which the site owner has not intended.
>
> For example on keymanweb.com, we really should match the desktop layout with the desktop keyboard even on iPad, if that's what the user has requested. And vice-versa.
>
> One solution may be to include a `deviceMode` parameter or similar for KeymanWeb:
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> `deviceMode = DEVICE_MODE.DETECT, DEVICE_MODE.TOUCH, DEVICE_MODE.DESKTOP`?
>
> * `DETECT` would be default and use the heuristics as per this PR
> * `TOUCH` would *always* show touch (and hence embedded would use this pathway)
> * `DESKTOP` would *always* show desktop
>
> Not sure what we have code-wise that already aligns with this but don't want to block off ways for site owners to use KeymanWeb in ways we may not have considered.
>
>

_Originally posted by @mcdurdin in https://github.com/keymanapp/keyman/pull/2268#issuecomment-548178449_

This primarily affects KeymanWeb (and its behavior when embedded in the iOS app).