[keyserver] make force login configurable

Summary:

• dedup'd UserCredentials in login.js and checks.js
• used forceLogin value from user_credentials.json in login.js

Depends on D11278

Test Plan: successfully logged in to identity from keyserver with forceLogin in user_credentials.json omitted, set to false, and set to true. tested that on the rust side, the correct value (None, Some(false), Some(true) respectively) was present each time

Reviewers: ashoat, inka

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D11285