Disable password reset and hide login form with SSO

[willbarton]

This PR does two things if SSO is enabled:

• Disables password resetting (via Wagtail's WAGTAIL_PASSWORD_RESET_ENABLED setting) and removes our password reset override template
• Hides the username/password login form behind an "Other ways to sign in" link on the login page

The second is accomplished by checking if a GET parameter (others) is provided when SSO is enabled, and if it is not, overriding the Wagtail login page's login_form and submit_buttons blocks.

How to test this PR

• Test without SSO enabled and ensure the login form displays correctly as usual, and the "Forgot password" link is available.

• Set the SSO environment variables listed in .env_SAMPLE. A test OIDC provider is not required for the login page to show the SSO button, and for the functionality of the "Other ways to sign in" link to be tested, but the values have to be defined for the site to run.

• Visit https://localhost:8000/admin

• Observe the username/password form is not displayed

• Click the "Other ways to sign in" link

• Observe the username/password form is displayed

• Observe that the "Forgot password" link is not shown

Screenshots

Checklist

• PR has an informative and human-readable title
• Changes are limited to a single goal (no scope creep)