Allow read-only spaces that represent document versions to be tied to GitHub tags

[rachellawson]

Currently, it is possible to attach a space to a GitHub branch and use different branches to represent different versions. However, that model could be improved by also allowing a space to be attached to a tag in GitHub. That space would then, by design, be read-only in GitBook.

This change would create a system that is more familiar for many users of code & documentation stored in GitHub as tags are more commonly used to represent releases.

If the tag is moved in the GitHUB repo, probably to represent a patch of some kind, then GitBook should detect this and update the space.