Implement API endpoints for returning top level measurement metadata

[hellais]

We would like to have an API endpoint for use by Explorer and Probe which takes as arguments an identifier of a measurements (report_id, input currently) and returns all the data needed to render the top level view.

This is for example in the case of a websites test:

{
"probe_asn": "AS1234",
"network_name": "Fidget Unlimited",
"probe_cc": "IT",
"test_name": "web_connectivity",
"measurement_start_time": "XXX",
"anomaly": true,
"confirmed": true,
"report_id": "XXX",
"input": "https://twitter.com",
"category_code": "SOCIAL",
"scores": {},
"analysis": {
   "blocking": "http-diff",
},
"failure": "",
"test_runtime": 2,
}

We should think about how to define the analysis key.

[sarathms]

I think it is useful to have some more data like:

* url to fetch raw measurement
* software version
* platform
* mk version

[FedericoCeratto]

ooni/api#166

[bassosimone]

This work is currently being worked on along with ooni/api#192.

[hellais]

Looks like this is done