Calculate the blocking key needed by OONI Explorer measurement pages

[hellais]

The web_connectivity measurement details page of OONI Explorer uses the test_keys->blocking key of the raw measurements (which is calculated on the probe) that can be mis-aligned with what the fastpath believes to be the blocking reason.

We should re-compute the blocking reason in the fastpath and serve it to OONI Explorer.

This issue superseeds: ooni/explorer#360

[azadi]

I am not sure if this is related but I will hopefully try to be useful:

The snippet below is from running a query on metadb:

   test_start_time   | measurement_start_time |              measurement_id               | m_report_no | anomaly | confirmed | coalesce_2 |                                                            scores                                                            | exc | residual_no |                                  report_id                                  | probe_cc | probe_asn |    test_name     |           input           
---------------------+------------------------+-------------------------------------------+-------------+---------+-----------+------------+------------------------------------------------------------------------------------------------------------------------------+-----+-------------+-----------------------------------------------------------------------------+----------+-----------+------------------+---------------------------
 2020-02-02 20:49:59 | 2020-02-02 20:49:59    | temp-fid-00d4fe139c14b9c6a86e3d5a0f6e16ff |    12411773 | f       | f         | f          | {"blocking_general":0.0,"blocking_global":0.0,"blocking_country":0.0,"blocking_isp":0.0,"blocking_local":0.0,"accuracy":0.0} | {0} |     1716728 | 20200202T204924Z_AS56402_FFVjCWvZqfWrhAZgXuJahKRGZjikgmUyJ5zerNdcuXmdepZHVo | IR       |     56402 | web_connectivity | https://ca.wikipedia.org/

But if you see the measurement on OONI Explorer (https://explorer.ooni.org/measurement/20200202T204924Z_AS56402_FFVjCWvZqfWrhAZgXuJahKRGZjikgmUyJ5zerNdcuXmdepZHVo?input=https%3A//ca.wikipedia.org/), it says that this is an anomaly and rightfully so.

Anyway, just thought I should share in case this was related.

[hellais]

@FedericoCeratto can you check if this is currently done?