[keyserver] Introduce isWebPlatform function

Summary: Step towards handling desktop device types, introduces isWebPlatform function that for now only checks if the platform is web but in the later diffs it will be expanded to also check for windows and macos. This diff shouldn't introduce any logic changes.

Test Plan: Search for all occurences of 'web' and platform that should be changed, run flow.

Reviewers: tomek, inka, kamil, ashoat

Reviewed By: ashoat

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D6888