[feature] Adds filtering by priority

[karlcow]

Feature:

1. Go to https://webcompat.com/issues
2. Select needsdiagnosis
3. Select Least Recently Updated ex: https://webcompat.com/issues?page=1&per_page=50&state=open&stage=needsdiagnosis&sort=updated&direction=asc
4. Feature: Show only priority-critical

This would diplay only the bug which priority-critical.

Risk, Technical issue:

The way we currently query the data is through pagination, we do not know the full corpus of issues. It means that we would need to make yet another request constraining the data set.

The technical issue can be lowered by having a local DB of all issues, probably something in between #788 and #165

[miketaylr]

Probably severity too.