feat: make full-text search case-insensitive and search by submission author as well

[Koc]

Follow-up for the #2710. Previously (before pagination introduction) we were able to find submissions by username just by pressing ctrl+f in a browser. Let's make it possible with server-side pagination as well to not degrade in functionality.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 45.48%. Comparing base (09d7d59) to head (bd2a73a).
Report is 10 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #2761      +/-   ##
============================================
- Coverage     45.58%   45.48%   -0.10%     
- Complexity      984      987       +3     
============================================
  Files            79       79              
  Lines          3565     3586      +21     
============================================
+ Hits           1625     1631       +6     
- Misses         1940     1955      +15     

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[Chartman123]

I think the search should always do the placeholder search for consistency. On the other hand, I'm not really sure if we should really extend the filtering to the username.

[Koc]

I've updated PR description with motivation of this change.