Allow document to be user scalable for the sake of a11y

[andyra]

Removes user-scalable=no from the viewport meta, which will allow people to scale as needed.

This was originally added in #1613 as a way to ensure that the screen doesn't zoom automatically in mobile Safari when text input sizes are < 16px. The aforementioned PR also forces the text size to be 16px on mobile Safari, though, so the user-scalable attribute is redundant and safe to remove.

https://app.fizzy.do/5986089/cards/3487