Set to default_pptx_template

[domwhewell-sage]

Closes #527
As mentioned in the linked issue the ArchiveView defaults to the default_docx_template by mistake when the report has no pptx_template configured

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 91.85%. Comparing base (e56d9e3) to head (fd43d01).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
ghostwriter/reporting/views.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #528   +/-   ##
=======================================
  Coverage   91.84%   91.85%           
=======================================
  Files         319      319           
  Lines       18573    18573           
=======================================
+ Hits        17059    17060    +1     
+ Misses       1514     1513    -1     

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[chrismaddalena]

Thanks for the catch and for submitting a PR!