Per-app config from ENV required for devserver

[jace]

Funnel provides multiple apps from a single module. These apps end up sharing configuration that is explicitly meant for a single app. Currently, a special config file hasgeekapp.py is used to isolate config for the main app, but this is a kludge.

We're migrating to config from environment variables. Since a worker (uwsgi) will be bound to only a specific app (currently app or shortlinkapp), it can supply app-specific config in the env and simply ignore the misconfiguration of other apps. However, this doesn't work for development where the apps are multiplexed via a single host app in funnel.devtest:AppByHostWsgi. They will again share a single configuration. Also, the app_url_for helper consults the config of unused apps, so misconfiguration is a problem there.

We can overcome this by using app-specific env prefixes. Flask defaults to FLASK_ prefix, which we can keep for common config, and then apply a second round of config using FLASK_{app_name}_ prefix. This feature needs to be exposed via Coaster's coaster.app.init_app.

[jace]

Implementation in #1732.