MDL-63401 tool_dataprivacy: Rewrite expired deletion handling

MDL-63401 tool_dataprivacy: Rewrite expired deletion handling

This change rewrites the way in which expiry is calculated and addresses
a number of closely related issues:

Users can customise, and add blocks with data to, their dashboard.  When
a user had done so, the user could be flagged for deletion, but the
blocks in their Dashboard were subject to the default block retention
policy. In addition there is no way to override the retention policy for
user blocks.

This change modifies the structure of the expiry mechanism:
- to consider any subcontext of the user context to be a part of the user
  context during calculation. User child contexts are not the property
  of the system, and should not be treated separately.
- the way in which the context expiry mechanism worked was to select
  use a multiple different managers based solely on the context level.
  Because of the use of user blocks, this proved to be unreliable as
  blocks has been attributed purely to courses.
  This has been changed to a single manager which is aware of hierarchy
  and deletions as a whole.
- to prepare for upcoming work relating to more detailed expiry
  mechanisms, a new expiry_info class is introduced and used to
  merge the expiry of child contexts into a working in-memory view.

This changeset includes extensive unit tests.