Use semantic versioning for cache busting

We are now using semantic versioning for the site. This provides us with an excellent opportunity to tie the site's versioning to situations where the number can be used for things like cachebusting our CSS.

Currently, the site uses a manual yyyymmdd format to accomplish this. It would be great if we could expose the semver number as a variable and use that instead. This way, updating the site's version also automatically updates caching, etc.