This issue is part of the MDL-62560 Epic.

- Assuming 1 would be a record that didn't exist. should be using 0.
- Missing space before AND in sql in provider.

This gets rid of specific repo functions which were unused, and makes
sure the following methods are defined on the interface, implemented
and tested:
- exists_by($criteria)
- find_by($criteria)
- delete_by($crtieria)
Also, added missing tests for find_favourite() repo method.

A template can include itself - e.g. by looping over the context children and rendering a tree.

Ensure this promise still resolves.

Example: admin/tool/lp/templates/competencies_tree.mustache.

This was using a helper, but now we have the function in core to achieve
the same thing, so use this instead.

Let's worry about reuse of the crud code later, when that requirement
arises, so moved that into the ifavourite_repository interface, and then
renamed it favourite_repository_interface.

Now: user_favourite_service (singular) and service_factory

Instead of favourites_repository we now have favourite_repository,
which is more in line with other implementations of this pattern.

This patch adds several things:
- favourite entity class under local/entity
- refactoring of repository and service to use the new class instead
of stdClass.
- update repository unit tests to use the object instead of stdClass.
- update service_test so that the mock repo requires the object for
the add operation.
- remove unnecessary constructor from favourites_repository class.