Intelligently filter out "monitoring disrupted" times

[Akii]

I noticed a major outage of monitoring where around 3k facilities were reported out of order with reason "monitoring disrupted". To fix this, we need to intelligently filter out disruptions:

1. Facilities that were previously disrupted should still be assumed disrupted even though monitoring is not available at the moment
2. Facilities that have been active prior to the monitoring outage should be assumed operational

As for calculating the grade and total downtime, there are basically a few cases of which only 2 are relevant:

|-------------------------|
|------------------------>| Case 1: Monitoring dis. only
|------------<----------->| Case 2: Disruption before, resolved after
|-----<------>------------| Case 3: Disruption before and after
|------------>------------| Case 4: Disrupted after

-> monitoring dis only = ignore
-> Case 2 -> we don't know when it really ended; have to include

[Akii]

As a quick solution, I've filtered out all "unknown" states from the DB source. This effectively eliminates like 80% of cases described here.

[Akii]

See #9