Factor out stuff into a Tails Python library

There’s some code duplication between at least Tails Greeter (feature/spoof-mac branch) and WhisperBack which should be eliminated by being refactored into a shared Tails Python library:

the cb_request_starting() callback used for the help browser
probably more of the help browser setup code (since it was pretty much copy-pasted from WhisperBack into TG)
__get_localised_doc_link() from WhisperBack (isn’t in TG yet, but will at some point)

Related issues

Related to #6786
Related to #6145
Related to #9372
Related to #11198
Related to #6525 (closed)

Original created by @anonym on 6452 (Redmine)

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