Use the measurement_uid instead of report_id, input tuple for linking to measurements

[hellais]

Since ooni/explorer#850, we list measurements via their measurement_uid instead of report_id, input tuple.

In the mobile (and desktop) apps, however, we don't use it yet.

As part of this issue we should add support for using the measurement_uid for referring to measurements. It might be necessary to make some changes to the DB tables in order to support this.

The measurement_uid should be returned as part of the measurement upload workflow, so when we upload a measurement we should retrieve it then and store it on the device in place of the report_id, input.

[hellais]

This is blocked by #2521