[keyserver] Use Platform instead of a DeviceType for sending notifs

Summary: We want to be able to send web notifications but currently send.js is typed using DeviceType which doesn't include web. This diff changes it to use the Platform type and changes some names to fit better. There should be no behaviour changes introduced in this diff.

Test Plan: Create a new account on iOS and check if notifications still work.

Reviewers: tomek, atul, kamil, ashoat

Reviewed By: ashoat

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D6813