Implement environment selector logic, make draft design for it.

Implemented logic includes:
* setting default environment on App Catalog page
* setting default environment upon pressing Add Application button at
  Environments->Services with immediate redirect to App Catalog page
* storing default environment in request.session

Implements: blueprint current-environment-selector

Change-Id: I1d98103efd645c6f1a3183679dee37c7d712486f
Merge "Implement environment selector logic, make draft design for it."