Add usage counters for images, documents and snippets
Merge branch 'master' into 146-display-calculated-usage

Conflicts:
	wagtail/wagtaildocs/models.py
	wagtail/wagtailimages/models.py
Merge branch 'master' into 146-display-calculated-usage
Add document usage count tests
'Usage' page now displays a table of pages. Add new 'USAGE_COUNT' setting.
Add unit tests for usage feature
Merge branch 'master' into 146-display-calculated-usage
Remove stray print statement
Change assertRegexMatches to assertRegex
Alter wagtailadmin.utils.used_by()

wagtailadmin.utils.used_by() no longer assumes that model
relationships to pages are named 'page'. Instead it assumes that all
ParentalKey relationships are relationships to pages.
Improve 'used by/usage count' implementation

- used_by() generates a database query instead of looping over a list of
  objects.
- usage_count() removed. Call count() on the queryset returned by
  used_by()
- Changed USAGE_COUNT to WAGTAIL_USAGE_COUNT_ENABLED. Added an inclusion
  tag for it.
Fix unit test failures
Merge branch 'master' into 146-display-calculated-usage
Fix Python 3 test failure
Add pagination to usage pages
Merge branch 'master' into 146-display-calculated-usage
Use wagtailadmin/shared/pagination_nav.html on usage pages
Merge branch 'master' into 146-display-calculated-usage

Conflicts:
	wagtail/wagtailimages/tests.py
Made useage count link translatable
Various renames, API changes, etc