Add a consent checkbox to the login page

[spoenemann]

Users accessing this EF service without an Eclipse account must explicitly accept our Terms of Use, Privacy Policy, and Code of Conduct before they can log in with another provider. This requirement does not apply to users authenticating via the Eclipse Foundation OpenID Connect service as we know they already accepted those. This can be implemented with a simple required checkbox that must be selected before proceeding with a third-party login.

Let's add simple JavaScript to disable the login buttons after page load and enable them when the checkbox is clicked.