Add `autocomplete=off` to dashboard product form

Fixes #631
Improve docs on how to customise an app

- Made instructions on using a custom template more expliciy.  Should be
  better for new users.
- Fixes a few typos and mistakes in the code examples.

This commit is squash of pull request #640.
Extend release notes with jQuery upgrade
[Backport] Fix bug with product class lookups in variants

This causes availability messages to fail in templates as an exception
is raised for variant products which don't define a product class.

Cherry-pick of 72eaafecb356032d198248e485a0fbcdfad5e3f2 back onto
releases branch.
[Backport] Handle group products in stockrecord template partial

Before this, group products were always shown as unavailable as they
didn't have a stockrecord - even though their variant products did.

Cherry-pick of f2922c9b3a63f035e20efae99a9fd3ba2e5c3b6d.
[Backport] Correct product URL on thank-you page

Cherry-pick of af6edf88ea419a4dc2e409ea61686f645c3ec19c
Release note for 0.5.1
Allow basket URL to be passed into JS init fn

This allows Django's reverse function to be used to correctly look up
the path for basket AJAX requests.

Fixes #644 - thanks to @jvanveen for spotting this issue.
Fix validation error in dashboard voucher form

The clean method was raising a KeyError when the date fields were empty.
Fix logic error in product availability

Introduced in 7fd62f2af078f950eeb278b97d01153b90b08e73.
Abstract stock alert should be abstract

The `AbstractStockAlert` was missing `abstract=True` in its meta,
causing two concrete models to be created.  The subclasssing
`StockAlert` model was nothing more than a pointer to the original
fortunately meaning the migration back to a single concrete stock alert
model is not too painful.

The migration drops the stockalert table and renames the
partner_abstractstockalert table to partner_stockalert.  This should
work fine and not drop any data.

Thanks to @Tilley for finding this pesky oversight in #645.
Add 'clean' target for removing ignored files
Update release note for 0.5.1
Bump version to 0.5.1
Merge branch 'releases/0.5' back into master

Conflicts:
	docs/source/howto/how_to_customise_an_app.rst
Bump version to 0.6-pre-alpha