Feat/is 585 govt sgid login rollout

[alexanderleegs]

Problem

This PR adds support for govt officials to log in via singpass. To be reviewed in conjunction with PR #1594 on the isomercms-frontend repo. It changes the existing sgin-login endpoint to return an email instead of just the status, and adds an additional endpoint to allow for users with multiple emails to call a separate endpoint to login after the initial sgid login step. It also modifies the existing behaviour of the add collaborators endpoint so that new users can be added as a collaborator directly instead of requiring them to log into the cms first.

Breaking Changes

• [x] Yes - this PR contains breaking changes
  • Minor breaking change - old sgid login will no longer work since the requested permissions no longer match. Acceptable behaviour because the existing sgid login is only usable by ogp officers currently

Tests

• [ ] Run tests on associated frontend PR
• [ ] Check that a collaborator that does not previously exist can be added to a site

[seaerchin]

Why would a user have separate emails?