Migration for recordingtimestamp (m2m -> 12m)

This is the first step, sanitizing the relationship to

Many recordingtimestamps -> One recording.
(previously m2m)
Initial form for editing timestamps

For example /recording/{N}/edit
This currently shows a formset of all data relating to timestamps,

Excludes unwanted fields for formsets.  Basic validation and updating.
Add default ordering to timestamps for correct display.
Switch RecordingUpdate to a DetailView.
Migration to add start_datetime and audio_duration
Additional validations using audio_duration and start_datetime
Add temporary link to recording edit page

(Unsure what UI preferences are on this, so currently very rough)
Set timestamp/duration on upload

We do the audio_duration setting in the model
(having first experimented in the view, which seems to be too late to
make the changes.  The form would also be a valid place to put this
behaviour, but seems to make most sense inside the model)
Testing duration/start_datetime setting