Versioning documents

[epatters]

It should be possible to version a document by (1) making a new snapshot of the document and (2) assigning it a version number/string.

Thus, a first step in this issue is to add an optional version column to the snapshots table. The key design questions are what should a version be and how opinionated should we be about what counts as a valid version. For example, a very restrictive scheme would make versions be numbers and require that they are consecutive integers starting at one. That seems too limiting to me. At the other end of the spectrum, a version could be an arbitrary string, with informal conventions/guidelines about how to choose them. That's how tags work in Git. In between might be some notion of "SemVer for models," though what that's supposed to mean isn't too clear.

Once that's decided, the task is to:

1. add RPC method to tag a new version, available to user's when Maintain or higher permissions on the document
2. expose this through a dialog in the frontend