Rely on database unique constraints

[jchristgit]

PR #245 updated the validators to check for duplicate usernames and
emails by running a query before posting the data. Let's rely on the
database indexes instead, which is more robust. This builds on top of
PR #245's tests to verify the functionality.

[0xAda]

LGTM!

[codecov]

Codecov Report

Merging #248 (83a6869) into master (f069a1e) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #248      +/-   ##
==========================================
+ Coverage   96.51%   96.55%   +0.04%     
==========================================
  Files         108      108              
  Lines        4966     4969       +3     
  Branches      265      264       -1     
==========================================
+ Hits         4793     4798       +5     
+ Misses        140      139       -1     
+ Partials       33       32       -1     

Impacted Files	Coverage Δ
src/authentication/basic_auth.py	100.00% <ø> (ø)
src/authentication/providers.py	100.00% <ø> (ø)
src/authentication/views.py	100.00% <100.00%> (ø)
src/team/serializers.py	100.00% <100.00%> (ø)
src/backend/exception_handler.py	81.08% <0.00%> (+5.40%)	⬆️

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