Allow preventing sending the notification email on GC creation

[Roshyo]

Fixes #205

[codecov]

Codecov Report

Merging #219 (7ec1891) into 0.12.x (5ae31ba) will increase coverage by 0.59%.
The diff coverage is 21.42%.

@@             Coverage Diff              @@
##             0.12.x     #219      +/-   ##
============================================
+ Coverage     49.73%   50.32%   +0.59%     
- Complexity      528      532       +4     
============================================
  Files           102      102              
  Lines          1687     1699      +12     
============================================
+ Hits            839      855      +16     
+ Misses          848      844       -4     

Impacted Files	Coverage Δ
src/Form/Type/GiftCardType.php	0.00% <0.00%> (ø)
...ener/SendEmailWithGiftCardToCustomerSubscriber.php	85.71% <100.00%> (+85.71%)	⬆️
src/Model/GiftCard.php	75.45% <100.00%> (+0.92%)	⬆️

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[loevgaard]

I think what this PR is missing is the case where an administrator updates a gift card, but doesn't change the email, then the user would get an email notification again.

This could easily be solved, I think, by just having the sendNotificationEmail property to be false by default