Improve date format heuristic for bank statement import

[kiancross]

When dates are ambiguous, use the event's locale to infer whether the day or month comes first. Since regions follow certain date format conventions, this helps make more accurate guesses.

This could probably be improved by (1) adding other countries to the check (although this can be extended later), or even better, (2) allowing the date format to be specified to avoid any wrong interpretations. However, I think this might be a reasonable stepping stone.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.43%. Comparing base (c4436ca) to head (4e03a89).
Report is 22 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4814   +/-   ##
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  Coverage   78.43%   78.43%           
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  Files         445      445           
  Lines       63063    63063           
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  Hits        49466    49466           
  Misses      13597    13597           

Files with missing lines	Coverage Δ
src/pretix/plugins/banktransfer/tasks.py	84.33% <100.00%> (ø)