Add query filter to applications index

The list of applications appearing in the apps/index view (the root view
for errbit) can be filtered using a query string such as
  `?apps[]=Appmazing!&apps[]=Super%20app`. This allows us to group
  applications relevant to specific teams.
Add date and active error filtering

Enables filtering of apps by problems `first_notice_at` date and apps
with unresolved errors.
Merge pull request #26 from alphagov/add_app_filtering_by_params

Add query filter to applications index