Social login UI polish (#9301)

changelog.d/9301.feature

synapse/handlers/auth.py

synapse/res/templates/sso.css

synapse/res/templates/sso_account_deactivated.html

synapse/res/templates/sso_auth_account_details.html

synapse/res/templates/sso_auth_bad_user.html

synapse/res/templates/sso_auth_confirm.html

synapse/res/templates/sso_auth_success.html

synapse/res/templates/sso_error.html

synapse/res/templates/sso_footer.html

synapse/res/templates/sso_login_idp_picker.html

synapse/res/templates/sso_new_user_consent.html

synapse/res/templates/sso_partial_profile.html

synapse/res/templates/sso_redirect_confirm.html

tests/rest/client/v1/test_login.py