restrict and correct nominal frequency values during tag registrations

[jbrzusto]

Some tag registration packages have been giving the funcubedongle listening frequency (e.g. 166.376) instead of the correct nominal frequency (e.g. 166.38) for a batch of tags. This causes the tag finder to select a much reduced set of possible tags to be searching for when it sees a radio tuned to the nominal frequency due to jbrzusto/find_tags#11. While that issue also needs to be fixed, we should still sanity check nominal frequencies supplied in tag registration packages.

[jbrzusto]

A set of tags was mistakenly registered to nominal frequency 166.376 MHz, and this has killed detections
of all other 166.38 MHz tags active during the same period. @cjardine-bsc has fixed the incorrect
nominal values, but we need to safeguard against a repeat.

[jbrzusto]

suggested fix:

• maintain a table of nominal frequencies in the metadata DB on sgdata.motus.org

• when a tag is registered, check the supplied nominal frequency against the table
  and complain fatally if it is more than 8 kHz off the closest match; otherwise, replace it with the closest match with a warning.

• add an API entry on sgdata.motus.org for query/add/remove of records
  in the above table (or just for query, and leave add/remove to admin
  users directly accessing the DB?)