ENH make event approval more robust to concurrent approval

[tomMoral]

Concurrent approval leads to server errors.
This avoids the error if another process already created the sandbox submission.

[codecov]

Codecov Report

Attention: Patch coverage is 71.42857% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 93.48%. Comparing base (97fbb52) to head (f4823ce).

Files	Patch %	Lines
ramp-database/ramp_database/tools/team.py	71.42%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #593      +/-   ##
==========================================
- Coverage   93.49%   93.48%   -0.02%     
==========================================
  Files         103      103              
  Lines        9015     9019       +4     
==========================================
+ Hits         8429     8431       +2     
- Misses        586      588       +2     

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