Add website and content version number to footer

[isaisabel]

As a user of the ATT&CK website, I want to be able to tell what version of the site I'm viewing.

The website and content (ATT&CK STIX) version numbers, as seen in the changelog, should be added to the website in the footer. It should be deemphasized using the on-color-deemphasis scss utility so that it does not draw undue attention.

[isaisabel]

The version numbers should be hyperlinks to the changelog.

[isaisabel]

Content version should be determined by the methodology described here. Website version should be in a new property in config.py. Until the content version is implemented in the STIX we should implement the content version as a property in config.py just like the website version.