Add Cypress tests and documentation

[chutch1122]

This PR addresses some things requested by the maintainer of NGINX Proxy Manager

• Cypress automated tests
• Documentation for setting up SSO with OIDC
• A more user-friendly error message for when a user does not exist in NPM and they are attempting to sign in via an IdP
• Fixing a typo in the "hint" description so that the name of "Redirect URL" matches the field label in the modal.

[chutch1122]

@oechsler I just added some additional UI-based end-to-end tests. These tests test logging in with a test user when OIDC is both enabled and disabled

[oechsler]

Hey @chutch1122, I completely missed the fact that you made further changes with your last push and merged them directly. If you think a review would still be beneficial, I can revert the changes and reopen this PR. Apologies for the mix-up on my part.

[chutch1122]

@oechsler Feel free to take a look at it. I'm just opened up a new PR anyway to merge in changes from the upstream develop branch. There were some changes to error messages at some point after the last time develop was pulled in that was causing the build to fail