fix: do not load historic in memory in clean_old_history command

[vied12]

uses query.count() instead of len(query) to count entries to be deleted

Description

clean_old_history command used to count entries to delete with len(). This loads in memory all the entries and can lead to crash in case memory limit is reached. Using a sql query to count to entries is faster and safer.

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 #735 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #735   +/-   ##
=======================================
  Coverage   97.44%   97.44%           
=======================================
  Files          18       18           
  Lines         980      980           
  Branches      148      148           
=======================================
  Hits          955      955           
  Misses         12       12           
  Partials       13       13           

Impacted Files	Coverage Δ
...e_history/management/commands/clean_old_history.py	100.00% <100.00%> (ø)

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[ThePumpingLemma]

Good catch!