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
Re-split audio on change of timestamps
Update start/end times on speeches as well as audio when timestamps are changed.
Ask for seconds since start of recording, rather than a datetime.
Refactor audio splitting into one recording update function.
Only have one function converting audio field to MP3 on model save.
Tests for modifying recording timestamp
Merge remote-tracking branch 'origin/recording-timestamp-modification-form' into recording-timestamp-modification-form
Test uses offsets instead of ISO date

as per matthew's fix
Better test data

using 3 timestamps confirms assumption that timestamp offset is
relative to the beginning (as opposed to the previous timestamp)
Allow deletion of timestamps
refactor timestamp tests to no longer inherit api tests