Add backend for filters

[SaptakS]

Description

• - Skills make it drop down select box (work with users or skills)
• - Bounties Completed - range
• - Leaderboard Rank - range

Refers/Fixes

Refs: #4220

Testing

[codecov]

Codecov Report

Merging #4299 into master will decrease coverage by <.01%.
The diff coverage is 33.33%.

@@            Coverage Diff            @@
##           master   #4299      +/-   ##
=========================================
- Coverage    30.2%   30.2%   -0.01%     
=========================================
  Files         209     209              
  Lines       16768   16776       +8     
  Branches     2242    2245       +3     
=========================================
+ Hits         5065    5067       +2     
- Misses      11516   11519       +3     
- Partials      187     190       +3

Impacted Files	Coverage Δ
app/dashboard/views.py	14.16% <33.33%> (+0.07%)	⬆️

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[octavioamu]

LGTM