Faster filtering on submission API v2 endpoint

[quantum5]

Instead of joining the user table and looking up the username, we can
simply look up the profile ID and use the index. A similar optimization
is done for problems. Fixes #1584.

This has been tested on the test site.

[codecov-commenter]

Codecov Report

Merging #1654 (c5347d5) into master (23fa853) will increase coverage by 1.04%.
The diff coverage is 51.61%.

@@            Coverage Diff             @@
##           master    #1654      +/-   ##
==========================================
+ Coverage   46.13%   47.17%   +1.04%     
==========================================
  Files         215      215              
  Lines       12228    12484     +256     
==========================================
+ Hits         5641     5889     +248     
- Misses       6587     6595       +8     

Impacted Files	Coverage Δ
judge/views/api/api_v2.py	47.77% <51.61%> (+0.74%)	⬆️
judge/models/tests/test_problem.py	100.00% <0.00%> (ø)
dmoj/settings.py	98.24% <0.00%> (+0.59%)	⬆️
judge/models/problem.py	92.94% <0.00%> (+4.35%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 23fa853...c5347d5. Read the comment docs.