Fix pagesets and complete CT e2e tests.

BUG=skia:3163

Review URL: https://codereview.chromium.org/821763002
- Adds backend code to filter by commits
- Adds first round of unit tests for Analyzer
- Fixes commit filter UI
- General refactor of query code to make it more extensible.

This sets us up to triage the HEAD digests quickly. Eliminating the need to triage everything.

The next CLs will extend the triage UI more to deal with commit ranges, add the status endpoint and add more filtering.

BUG=skia:2930

Review URL: https://codereview.chromium.org/799623002