Add warning when posting code to comments

[int-y1]

trafficstars

cleaned-up version of https://github.com/VNOI-Admin/OJ/pull/201

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 46.78%. Comparing base (fd7fb05) to head (c3d0cab). Report is 14 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2343      +/-   ##
==========================================
+ Coverage   46.76%   46.78%   +0.01%     
==========================================
  Files         251      252       +1     
  Lines       13317    13348      +31     
==========================================
+ Hits         6228     6245      +17     
- Misses       7089     7103      +14     

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[Ninjaclasher]

I think this code is also used for contest / blog post comments. Maybe we want a different message there since redirecting to the "submit solution" button is no longer meaningful.

[int-y1]

list of changes:

• popup message changed. "post a source code" -> "post some source code"
• popup only appears on /problem/<problem code>. all other comment sections still work, and aren't affected.