Support permalinks with trailing slash removed

[jakejackson1]

Description

Resolves #1489 (although we didn't have a problem accessing signed PDF URLs under this configuration)

Testing instructions

1. Enable Pretty Permalinks in the WordPress settings
2. Remove the trailing / from the custom permalinks section
3. View a PDF from the admin area

Screenshots

Checklist:

• I've tested the code.
• My code is easy to read, follow, and understand
• My code follows the accessibility standards.
• My code has proper inline documentation / docblocks.

Additional Comments

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (807bc8e) 75.19% compared to head (5b4381b) 93.30%.

Additional details and impacted files

@@                Coverage Diff                 @@
##             development    #1495       +/-   ##
==================================================
+ Coverage          75.19%   93.30%   +18.11%     
==================================================
  Files                243       57      -186     
  Lines              12939     1464    -11475     
  Branches             370      370               
==================================================
- Hits                9729     1366     -8363     
+ Misses              3202       90     -3112     
  Partials               8        8               

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