Subject Preferences in Segue UsersFacade not Isaac Class

[jsharkey13]

The lines that persist user subject preferences are currently in a Segue class, despite being a very Isaac specific thing.

A sensible fix may be to move the actual users endpoint to a new Isaac UsersFacade, which then uses the base Segue methods for the RegisteredUserDTO parts, but can safely separate out the preferences in the Isaac world . . .
(This does mean more classes though . . .)