Get default user for bulk ops from middleware

[rossmechanic]

Description

Bulk utility operations such as bulk_create_with_history and bulk_update_with_history were recently added to the library. Both functions allow you to specify a default user, or to specify the user on the object instances themselves using _history_user. However, there's no easy way to get the current user in the same way that we get it for normal historical model creation. This PR updates those bulk create operations such that the user is the same user that would be applied if they were not bulk operations (unless you specify

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have run the make format command to format my code
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have added my name and/or github handle to AUTHORS.rst
• I have added my change to CHANGES.rst
• All new and existing tests passed.

[codecov]

Codecov Report

Merging #677 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #677   +/-   ##
=======================================
  Coverage   97.23%   97.24%           
=======================================
  Files          18       18           
  Lines         977      980    +3     
  Branches      147      147           
=======================================
+ Hits          950      953    +3     
  Misses         13       13           
  Partials       14       14           

Impacted Files	Coverage Δ
simple_history/manager.py	97.36% <100.00%> (+0.03%)	⬆️
simple_history/models.py	98.46% <100.00%> (+<0.01%)	⬆️

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