No longer possible to index an abstract model

Also a bit of cleanup to centralise all "can this model be indexed?"
logic
Treat MPO formatted images like JPEG. Fixes #729
release note for #676
remove comment that refers to logic now done in get_image_edit_form
Merge pull request #716 from kaedroho/search/prevent-abstract-models-from-being-indexed

Search: Prevent abstract models from being indexed
release note for #716
Merge branch 'issue-729' of https://github.com/kaedroho/wagtail into kaedroho-issue-729
Merge branch 'kaedroho-issue-729'
release note for #730
Fixed comment in images/views/multiple
Removed b'' prefix from Django 1.7 migration

These cause issues under Python 3. Please update Django
Added failing test for #756
Don't clobber latest_revision_created_at

Fixes #756

This field was being set to None whenever a revision was pushed live
Added failing test for #735
Don't update descendant urls if slug not updated

Fixes #735

Previously, if you called Page.save with update_fields and didn't include
'slug', it would still attempt to update URL paths of descendant pages.

Wagtail 0.7 introduced a feature where it saves the latest revisions
created at date on the Page. This requires a Page.save call which used
update_fields. The bug caused update_descendant_paths to be called
anyway even though the slug wasn't committed to the database.

This caused Issue 735 because publishing a page will both save a
revision and save a page causing two hits to update_descendant_paths.
Running this method twice caused all descendant pages url_path
attributes to get mangled.
Unit test for form-builder forms with multiple checkboxes
Use form.cleaned_data when processing submissions

This fixes #584. It also doesn't pack single values into lists and will
remove csrftoken for us.
release note for #768
Merge branch 'kaedroho-mope-584-fix-formbuilder-checkboxes'
Merge pull request #769 from kaedroho/issue-735

Don't update descendant urls if slug not updated. Fixes #735