Preserve trailing underscores

[tieneupin]

Encountered an edge case today whereby our use of werkzeug.utils.secure_filename() removed trailing underscores from file path parts, which we should guard against.

This PR streamlines the logic of murfey.utils.__init__.secure_path() and adds safeguards against removing trailing underscores in a file path part. Unit tests were also added to ensure file renaming occurs as expected.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 34.40%. Comparing base (24d6628) to head (738c187).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #676      +/-   ##
==========================================
+ Coverage   34.32%   34.40%   +0.07%     
==========================================
  Files          92       92              
  Lines       11157    11158       +1     
  Branches     1474     1474              
==========================================
+ Hits         3830     3839       +9     
+ Misses       7202     7195       -7     
+ Partials      125      124       -1     

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