Auto extend SAML SP metadata validUntil date

[collinpreston]

Bootcamps functions as a SAML IDP for a single SP. The SAML IDP library that we use in bootcamps expects that every SP metadata has a validUntil attribute, which unfortunately many SP's don't include.

If the current date is past (larger) than the validUntil value in the metadata then bootcamps will no longer function as an IDP for that SP since the metadata is considered expired.

In order to work around the lack of a validUntil attribute or expired metadata breaking our IDP service, we should add the environment variable shown here. In order to use the functionality shown in that referenced PR we must first ensure that our version of djangosaml2idp includes that PR.