Add extra context to admin

[barm]

Adopts #467

add extra_context to history_form_view

Description

Related Issue

Motivation and Context

How Has This Been Tested?

Screenshots (if appropriate):

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:

• My code follows the code style of this project.
• 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.
• All new and existing tests passed.

[codecov-io]

Codecov Report

Merging #499 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #499      +/-   ##
==========================================
+ Coverage   97.22%   97.22%   +<.01%     
==========================================
  Files          16       16              
  Lines         720      721       +1     
  Branches       95       95              
==========================================
+ Hits          700      701       +1     
  Misses         10       10              
  Partials       10       10

Impacted Files	Coverage Δ
simple_history/admin.py	98% <100%> (+0.02%)	⬆️

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