Move _users _design/auth into Erlang

Right now we enforce `_users` to have a `_design/auth` ddoc with a VDU in it that we force the value of.

## Expected Behavior
_users VDU should be hard-coded in Erlang and enforce the same rules as the JS VDU today.

## Current Behavior
We forcibly create `_users/_design/auth` and forcibly maintain the contents of its VDU.

## Context
This would be a prerequisite towards an optional no-couchjs version of CouchDB 2.x, something we discussed in https://github.com/janl/couchdb-next/issues/23 .

Start by examining how CouchDB currently creates and maintains _users/_design/auth and how its JavaScript VDU enforces the existing rules. Trace the corresponding Erlang request or validation entry point; done means the same _users VDU rules are enforced in Erlang without forcibly creating or maintaining the design document.

Written by the indexing model from the issue text.