Add filter for "Anomalies" on MAT

[agrabeli]

Similarly to the OONI Explorer Search tool (https://explorer.ooni.org/search?since=2022-01-24&until=2022-02-24&failure=false&only=anomalies), it would be great if the MAT also included a tool for filtering by anomalies.

This would make it much easier when trying to answer questions like "In which countries is WhatsApp potentially blocked?", "In which countries is bbc.com potentially blocked?", etc. And these are probably the single most important questions that we always ask when looking at OONI data.

I therefore consider it high priority to add an "Anomalies" filter, since currently it requires scrolling through endless country charts trying to spot orange bars, which is quite tedious.