Remove redundant code from to get contenttype and load content in the Content Bank

In several places, the Content Bank code checks the DB for specific content and then invoke a specific contenttype_XXX\contenttype and  contenttype_XXX\content to operate the methods.

This code is always the same and now it is encapsulated into the methods:

• contentbank\content::get_content_type_instance
• contentbank\contentbank::get_content_from_id

For example, in functions such delete_contents, move_contents, rename_content, etc.