Merge pull request #2897 from tomds/static_placeholders

Fix error when publishing static placeholders outside of a CMS page
fixes #2286
cleanup
added inherit to api check
Merge pull request #2898 from digi604/fix-2286

fixes #2286 (api validates templates)
Isolates the __page_lookup cache from other projects using the same cache backend
Merge pull request #2899 from mkoistinen/page_lookup_cache

Isolates the __page_lookup cache from other projects using the same cache backend