[web] add identity password login to traditional-login-form.react.js

Summary:
can now successfully log in to the identity service from web with username/password if usingCommServicesAccessToken is enabled

registering users reserved on ashoat's keyserver will be handled in ENG-4033 and will be transparent to the user, so no changes will be needed here

Depends on D10817

Test Plan: successfully logged in to identity service from web modal. also tested that the correct error message is displayed if user isn't registered

Reviewers: ashoat, inka, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D10821