[SECURITY][Bugfix:Forum] Fix Forum Links and Deletion

[cjreed121]

What is the current behavior?

Threads/posts could be deleted by a different user. Also it is possible to create links that could lead to XSS.

What is the new behavior?

Threads/posts can only be deleted by the author or an authorized user. Links are checked in the markdown renderer so XSS is not possible.

Other information?

Thanks Fu (@richcfno1) for pointing these out.

[codecov]

Codecov Report

Merging #8032 (c5d7afe) into main (ceb26e9) will increase coverage by 0.29%.
The diff coverage is 33.33%.

@@             Coverage Diff              @@
##               main    #8032      +/-   ##
============================================
+ Coverage     22.12%   22.41%   +0.29%     
- Complexity     7569     7715     +146     
============================================
  Files           203      203              
  Lines         24734    25023     +289     
  Branches         60       60              
============================================
+ Hits           5472     5609     +137     
- Misses        19204    19356     +152     
  Partials         58       58              

Flag	Coverage Δ
autograder	20.22% <ø> (ø)
js	31.34% <ø> (ø)
migrator	99.20% <ø> (+<0.01%)	⬆️
php	20.59% <33.33%> (+0.35%)	⬆️
python_submitty_utils	71.65% <ø> (ø)

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