Unify Version Slugs

Currently we identify the version of documentation that a user wishes to see with a version slug. Under the hood these are of the form `boost-1-81-0`. However, we have a number of different places and ways in which we actually display/retrieve the current version, such as in the library documentation url where it is of the form `1_81_0`.

This has lead to at least one bug in the form of #1993 , and will inevitably lead to more.

### Requirement

Expand the utility function in libraries `modernize_boost_slug` to handle all of the forms that a slug may be passed to the backend and convert it to the standardized back end form. We should write a regex for each form and a converter function that then gets called in a switch statement inside the modernize function.

Additionally, we should add a verification process to the slug field on Versions to ensure that they are entered consistently.