- Rework view to use simpler methods
- Rename reloading method (not sure why, for readability I think)
- Use a form for changing the order status (we should be using forms for
  changing line stuff too - will look at that next)
- Generally clean up the view a little

Motivated by #988, this change adds extra validation for when quantities
are submitted on the dashboard page. Without this, negative and too-high
values can be submitted without getting blocked by validation checks.

Fixes #988

This should always be used instead of Django's silent version.

Tests failed on a few occasions, because CharFields were set to None or
expected to be None.

I had thought we wouldn't need this but it turns out you do in order to
consume/cancel stock allocations through the order EventHandler.

- Only handles Datacash payments at the moment

This is useful for linking between payment events and transactions.

- Add more links
- Add event details to line detail page

This is for checking if any line has passed a given shipping event.

This is useful when validating potential shipping events.

when trying to perform order management events.