Use version numbers instead of names with previous versions

[isaisabel]

As a user of ATT&CK, I want to be able to visit previous versions of the site by their version numbers instead of names. In other words, I want to be able to visit /v5/ instead of /july2019/.

Currently deployed previous versions should be updated with redirects to their new version-numbered pages.

[isaisabel]

• Even though we have minor content versions (e.g v5.1, v5.2), we only preserve each major version of the site. The major version preserved would be the same as the highest patch version, e.g /v5/ would correspond to v5.3 and v5.2, v5.1 and v5.0 wouldn't be preserved.
• The version number used is the content version number, not the website source code version number.