relax url regexes to allow frontend editing of models with non numeric pks
Extract regexp for slugs as a reusable constant
Reuse SLUG_REGEXP in FrontendEditableAdminMixin
Add test for FrontendEditableAdminMixin.edit_fields url matches
Add placeholder and view to CharPksExample
Move FrontendEditableAdminMixinTest to cms.tests.toolbar. Add View tests.
Rename FrontendEditableAdminMixinTest to CharPkFrontendPlaceholderAdminTest
fix tests, migrations
Merge pull request #3597 from yakky/feature/frontend_admin_regexp

Support non-numeric pks in FrontendEditableAdminMixin / PlaceholderAdminMixin
Fixes #3578