Switch from g.user to current_user

[jace]

Coaster currently expects a g.user entity in load_models. If the implementation for hasgeek/baseframe#80 (CRUD views) is moved to Coaster, that will add a second reference to g.user.

However, hasgeek/flask-lastuser#24 proposes replacing g.user with a current_user proxy, which must be explicitly imported. This is a problem because Flask-Lastuser depends on Coaster. Coaster can't import from Flask-Lastuser.

We can get around this cyclic dependency by making a choice:

1. Stick with g.user. The problems it causes will remain, such as the possibility of an app's before_request handler attempting to access it before it is set.

2. Coaster should add a dependency on Flask-Login and import current_user from there. Flask-Lastuser will need to plugin to Flask-Login and replace the user loading mechanism there.

3. Coaster should provide its own current_user proxy, but allow the backing implementation to be replaced (process-wide). Importing Flask-Lastuser into the same process will automatically apply this replacement.

[jace]

Choice 1 is clearly not viable any further. Flask-Login makes assumptions about users that aren't necessarily compatible with Lastuser, so we should not add a dependency to it.

If we take choice 3, we need to demonstrate that Coaster's current_user proxy can be compatible with both Flask-Login and Flask-Lastuser.

[jace]

Discussion on implementation details is in #155.