Make init_for calls automatic

[jace]

hgapp-based apps currently require init_for to be called explicitly. The Flask extension ecosystem however expects init to happen implicitly during module load and often provides no hook for config to be loaded.

We should switch to implicit config by (a) reading an environment variable named FLASK_ENV for the expected environment (default dev) and (b) call it at the bottom of __init__.py itself, instead of having an external caller (runserver.py, website.py, etc).

Coaster's manage.py expects env and init_for parameters. Those must be removed. This therefore requires a coordinated switch across all repositories.

Production deployments must have export FLASK_ENV="production" specified somewhere globally, like in /etc/rc.local.

[jace]

The Flask-Environments extension, which does something similar, uses DEVELOPMENT as the default value. We currently expect lowercase in app.py, but (a) we can switch to case insensitive, and (b) for the sake of compatibility, specify upper case DEVELOPMENT as the default value.

Further, defaults should be specified in a single place, so this should be in the init_app function.

[jace]

Please file issues in all apps with the same title and the following text:

Refer to hasgeek/hgapp#6

[jace]

If we move the parsing of FLASK_ENV into Coaster, the init_for function itself is unnecessary and all config can move inline.