[identity] get reserved usernames from JSON

Summary:
copied over the list of reserved usernames from reserved-users.js in lib.
since the js file is hardly ever updated and we'll probably remove it soon, i figure it's fine to have two lists for now.

if anyone feels strongly, though, i can refactor reserved-users.js to use this JSON file

Depends on D7531

Test Plan: tested as part of register user workflow in later diff

Reviewers: ashoat, jon, bartek

Reviewed By: jon

Subscribers: tomek, atul

Differential Revision: https://phab.comm.dev/D7532