Remove the "ShopEmail" injector setting.

[bummzack]

Change the OrderEmailNotifier accordingly.
Change debug output of emails, so that the ShopEmailPreviewTask is usable.

[codecov-io]

Codecov Report

Merging #641 into master will decrease coverage by 0.08%.
The diff coverage is 50%.

@@             Coverage Diff              @@
##             master     #641      +/-   ##
============================================
- Coverage     46.97%   46.89%   -0.09%     
- Complexity     1561     1564       +3     
============================================
  Files           115      115              
  Lines          4534     4544      +10     
============================================
+ Hits           2130     2131       +1     
- Misses         2404     2413       +9

Impacted Files	Coverage Δ	Complexity Δ
src/Checkout/OrderEmailNotifier.php	70.58% <50%> (-6.59%)	20 <1> (+3)

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