Show centre frequency on the saved spectrograms

[tstranex]

On the saved waterfall plot (the one with - & + kHz legends, that's tied to a successful pass, etc.) add the center frequency to the legend at the very top (with name, UTC, etc.).

The code that sets the title is SpectrogramTitle in src/carpcomm/demod/spectrogram.go

When the contact has a satellite attached, we can lookup the frequency via the satellite_id. However, that doesn't work when no satellite is attached. What we need to do is add a centre_frequency_hz field to the Contact proto.

Relevant code: src/carpcomm/pb/contact.proto NewContact in src/carpcomm/streamer/contacts/populate.go SpectrogramTitle in src/carpcomm/demod/spectrogram.go