Move Flask-Lastuser integration into Baseframe as an optional module

[jace]

HasGeek apps currently depend on Flask-Lastuser, but in the hgapp structure of all apps, Baseframe (which handles UI) and Flask-Lastuser (which handles auth) have no explicit awareness of each other. Baseframe however does refer to the g.user object which is expected to be a Lastuser User object.

All code related to Flask-Lastuser integration in the app is in the hgapp skeleton, which makes it difficult to make upgrades as patches have to be applied manually to all hgapp-based apps. Baseframe has started adding functionality to apps since it acquired an init_app method -- most recently cache and assets and some awareness of the Lastuser extension -- but still no explicit handling for Flask-Lastuser.

This should change. Flask-Lastuser should be provided by Baseframe, including the boilerplate methods in login.py.