Using standard subplugin support, this commit implements
the backup of logstore subplugins in general and the
standard logstore in particular. Notes:
- Uses a custom final element (base64_encode_final_element) to
support the storage of serialized 'other' information in logs.
- Organization: Instead of directly extending backup_subplugin,
every logstore extends backup_tool_log_logstore_subplugin just
in case any shared code is needed in the future.
- Implements both course and activity logs, sharing the structure
completely (both are based in contextid to pick the target
information, from database or whatever other logstores use).
Now users can choose to exclude the question bank from the backup,
this will in turn prevent any modules which use the question bank
from being included in the backup.
