Refactor version navigation logic

This diffset aims to move logic powering the version navigator out of HTML / JavaScript and into the configuration for each doc's repository. 

`docs_meta.yaml` for both docs and mms_docs now requires a new variable, `conf.version.base`, which defines the first segment of the destination url.

Some edge cases are probably incorrect at the moment, but I think the easiest way to push through this refactor is for Sam and I to work together on working out the kinks.