Settings: validate public gateway before saving it

[lidel]

Current state

#1834 introduced ability to customize public gateway used for shareable URLs and download via preview screen.
The only validation we do is making sure the entered string is a valid URL.

Proposed improvement

We should validate that the gateway is functional before storing it in localStorage to avoid users breaking their experience by entering an invalid gateway address (typo, server being down, etc).

Note on how to validate

Some gateways may not allow cross-origin requests (CORS), so we can't test them using regular fetch.
Workaround: use Image load instead.<img> tags are not being validated using CORS rules.

Prior art is checkViaImgSrc – can be found here.