Moved FixBaseclassesTest tor test_migration_browser.py

This is easier to find  promots cleaner separation.
cherry pick migration-form
refactor migration and test for schemaextended fields
use view for migration, reuse vocab for schemaextended types
compare schemata and check for interfaces to find schemaextended
also migrate blobs
hide migration-form when there is nothing to migrate
add PLONE_APP_CONTENTTYPES_MIGRATION_TESTING layer.

This layer only installs Products.ATContenttypes.
It's used to have a proper environment to test migration
to plone.app.contenttypes.
add ATBlob tests and use migration layer for test_migration
Add tests for vocabularies used for the migration
cleanup tests: Products.ATContenttypes has no gs profile to run at all
Merge branch 'master' into migration_refactore_tests

Conflicts:
	plone/app/contenttypes/testing.py
	plone/app/contenttypes/tests/test_migration.py
Fix regression: plone.app.contenttypes.interfaces.IEvent moved
Add AT object numbers ; estimate migration time
cleanup tests
Fix regression: migration tests do not work

due to missing 'Products.DateRecurringIndex
return some results when using the migration-form
Merge branch 'migration_form' into add-vocab-tests

Conflicts:
	plone/app/contenttypes/tests/test_migration.py
Merge branch 'master' into migration_form

Conflicts:
	plone/app/contenttypes/tests/test_migration.py
update docs