Allow site enabledFrom and enabledTo dates to be null; Added site selector unit tests.

This code change allows a site's enabledFrom and enabledTo dates to be null.

It may be desirable to not have to worry about the enabledFrom and enabledTo dates at all, rather than having to extend them every 5 years, when everyone involved with the site has likely forgotten about the need to extend the enabledTo date.

This is a new pull request to address issues I ran into with the previous pull request that had  merge conflicts that was giving me fits.

Thanks!