Add Mozilla Location Services (MLS) support

Created by: E3V3A

Issue: This is not an issue, but would be a great improvement to the flexibility of our BTS data sources.

We are currently depending on the OpenCellID (OCID) DB to obtain the BTS info we need for our detection. This data is used to populate the DBe table. However, Mozilla Location Services (MLS) is now mature, detailed and large enough for us to benefit from their carefully curated data. It is maintained on GitHub as ichnaea.

We already have a very long issue thread in issue #71. It is too long and not enough specific, which is why this issue is created. We already have much of the framework for getting data, so we need to add a setting as in this comment:

External BTS data source(s): 
( ) OCID only
( ) MLS only
(o) both OCID and MLS

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