Use latest_revision_created_at in sitemap gen

Much faster than looking it up manually for every page in the site
Added failing test
created_at of PageRevisions is now copied on page copy
Added Django 1.7 migration
Project template: Fixed migration dependency

The project template currently crashes on first migrate because it tries
to run the create homepage migration before the Page.locked field is
created.

This commit fixes (and hopefully, future proofs) this by telling Django
that the core app migrations depend on the latest wagtailcore migration,
which should make Django fully migrate wagtailcore before starting the
core app.
Fix revisions still copying for child pages

When Page.copy runs recursively, it forgot to pass through the
copy_revisions value that the user specified causing revisions to always
be copied for subpages regardless of what the user wanted
Merge pull request #732 from kaedroho/copy-revisions-created-at-bug

Don't overwrite created_at of copied page revisions
release note for #732
Merge pull request #748 from kaedroho/fix-revisions-copying-on-child-pages

Fix revisions still copying for child pages
Release note for #748
Merge pull request #720 from kaedroho/use-lastest_revision_created_at-in-sitemap-generator

Use latest_revision_created_at in sitemap generator
Merge pull request #743 from kaedroho/template-dependency-fix

Project template: Fixed migration dependency
Release note for #743
Reorganised images tests
Merge pull request #751 from kaedroho/images-tests-reorg-2

Reorganised images tests
Removed PageQuerySet.unpublish method. Fixes #745