Incorporate feedback for virtual timeslots

See merge request !57

API consumers must not be tempted to parse ids, so we encode them as
UUIDs to prevent that.

Up until now the id of the first and last program entry was based on the
filtered range. Instead, we want these to be based on the first timeslot
that was scheduled before/after the specified range. This way we ensure
that the ids of these entries are stable as long as the schedule doesn’t
change.

Mapping keys in filter_data are guaranteed to be defined but might be
None for boolean filters. We want to ensure that we actually pass
booleans to generate_program_entries.

This flag allows clients to define what they expect as default behaviour
for setting the ProgramEntry start and end attributes.

If the flag is set steering ensures that ProgramEntry.start never starts
before the requested start and that ProgramEntry.end never ends after
the requested end.

We might need that queryset unbothered by already set filters later on
;)

Add debug application-state endpoint

See merge request !56

This new endpoint allows clients to easily purge steering’s application
state. It is primarily designed for use in integration tests as outlined
in aura#387.

The endpoint is available in debug mode so that developers can inspect
the schema. However, any request to it is denied unless the
OPERATING_MODE environment variable is set to "tests". This is a
precaution for administrators that use the debug mode to analyze
application issues in production.