add validator to check entity uniqueness in db

[codecov[bot]]

Codecov Report

Merging #71 (491326e) into master (c2463ec) will increase coverage by 0.05%. The diff coverage is 97.87%.

@@             Coverage Diff              @@
##             master      #71      +/-   ##
============================================
+ Coverage     96.66%   96.72%   +0.05%     
- Complexity      147      180      +33     
============================================
  Files            27       30       +3     
  Lines           390      458      +68     
============================================
+ Hits            377      443      +66     
- Misses           13       15       +2     

Impacted Files	Coverage Δ
src/Validator/Constraints/MatrixRecordUnique.php	100.00% <ø> (+20.00%)	:arrow_up:
...ator/Constraints/DatabaseEntityUniqueValidator.php	96.92% <96.92%> (ø)
src/Validator/Constraints/AbstractValidator.php	100.00% <100.00%> (ø)
src/Validator/Constraints/DatabaseEntityUnique.php	100.00% <100.00%> (ø)
...idator/Constraints/MatrixRecordUniqueValidator.php	100.00% <100.00%> (ø)
...odel/Configuration/AbstractImportConfiguration.php	91.83% <0.00%> (-4.09%)	:arrow_down:
src/Controller/BaseImportControllerTrait.php	90.76% <0.00%> (-0.14%)	:arrow_down:
src/Form/Type/MatrixType.php	100.00% <0.00%> (ø)
src/Form/Type/MatrixRecordType.php	100.00% <0.00%> (ø)
src/Controller/ImportControllerTrait.php	100.00% <0.00%> (ø)

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