Allow upload of submission PDF

[chrispyles]

Closes #764

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (10d6dec) 82.06% compared to head (55f2c4a) 82.15%.
Report is 1 commits behind head on master.

Files	Patch %	Lines
...tter/run/run_autograder/runners/abstract_runner.py	77.77%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #765      +/-   ##
==========================================
+ Coverage   82.06%   82.15%   +0.08%     
==========================================
  Files          64       64              
  Lines        4378     4387       +9     
==========================================
+ Hits         3593     3604      +11     
+ Misses        785      783       -2     

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