Some artefact refactoring (Bug #1298646)

Fixes in this patch:

- Moved artefact.php to artefacts directory to separate it from pages.
- Fixed reference to a wrong 'artefactonlyviewableinview' string.
- Removed add_to_render_path() and its calls which have no purpose at all.
- Removed 'artefact_parent_cache' table.
- Removed cron jobs related to 'artefact_parent_cache' from DB.
- Added 'path' column in 'artefact' table to easier calculate hierarchy.
- Added ArtefactTest.php for artefacts unit tests

Change-Id: Ia14cd85b94c32a950354446ee3565bd2964c625c
Signed-off-by: Yuliya Bozhko <yuliya.bozhko@totaralms.com>
Merge "Some artefact refactoring (Bug #1298646)"