Added ability to configure width of evidence images

[reesercollins]

Issue

#596

Description of the Change

All current evidence inserting into report documents are 6.5 inches wide, however certain templates may not be suited to 6.5 inch wide images. The current recommendation is "you can change the size after the report is generated". This change allows users to configure report templates to set the size of inserted evidence.

Alternate Designs

An alternative solution would be to add a global report configuration option to set the width of inserted images, however this is not ideal when different templates may look best with different width images.

Possible Drawbacks

None I can foresee

Verification Process

I tested the change by exporting reports before and after changing the "evidence image width" value. The reports that were downloaded shows the images are being correctly set to the desired width.

Release Notes

Added per-template configurable width for embedded evidence images

[chrismaddalena]

Thanks for submitting this! The code changes and approach look good to me. I like the idea. I'll test it out today.

[codecov]

Codecov Report

Attention: Patch coverage is 81.81818% with 2 lines in your changes missing coverage. Please review.

Project coverage is 92.07%. Comparing base (b730ba9) to head (be2e89a).
Report is 6 commits behind head on master.

Files with missing lines	Patch %	Lines
ghostwriter/reporting/models.py	33.33%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #597   +/-   ##
=======================================
  Coverage   92.07%   92.07%           
=======================================
  Files         324      325    +1     
  Lines       19061    19069    +8     
=======================================
+ Hits        17550    17558    +8     
  Misses       1511     1511           

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