Add email notifications for users assigned to categories

Users who are assigned to categories can now receive email
notifications. This is configurable via E-mail Notifications page,
on a per-project basis.

A common use case:

• A user was assigned to some category;
• When new issue is submitted to this category and "Assigned To" field
  is empty, this issue is assigned to Category Owner (who receives a
  notification);
• If we then re-assign issue to someone else, the Category Owner will no
  longer receive notifications.

This commit makes it possible for category owners receiving
notifications in this case as well.

Fixes 0020876

Signed-off-by: Damien Regad dregad@mantisbt.org

Original commit amended to default the new flag to ON as per [1].
Reworded and formatted commit message.

[1] https://github.com/mantisbt/mantisbt/pull/762#issuecomment-217280123