dashboard/app: hide useless coverage signals

[tarasmadan]

Is your feature request related to a problem? Please describe.
Coverage page has too many numbers.
It makes sense to hide or highlight numbers using additional criteria.

Describe the solution you'd like
Additional use-cases to consider are:

1. Hide files/dirs with 0 coverage signals.
2. Hide well-covered files/dirs (with 100% only or >threshold).
3. Sort items in every dir by instrumented_count. It allows to focus on the most impactful lines.

Additional context
@dvyukov asked to reuse percent.