FEAT: Add new VisibleToCustomer flag to OrderStatusLog

[madmatt]

Should be backwards-compatible. Includes an upgrade task that correctly sets the flag for old records

• Add new boolean flag VisibleToCustomer to OrderStatusLog
• Add requireDefaultRecords to migrate and upsert the new VisibleToCustomer flag to existing records
• Add description for existing SentToCustomer field (after discussion on it's purpose with wilr) and VisibleToCustomer
• Set new flag appropriately in existing logic
• Add note to changelog for an upcoming (unreleased) version

[codecov-io]

Codecov Report

Merging #736 into master will decrease coverage by 0.14%. The diff coverage is 26.66%.

@@             Coverage Diff              @@
##             master     #736      +/-   ##
============================================
- Coverage     46.08%   45.94%   -0.15%     
- Complexity     1568     1572       +4     
============================================
  Files           115      115              
  Lines          4550     4575      +25     
============================================
+ Hits           2097     2102       +5     
- Misses         2453     2473      +20

Impacted Files	Coverage Δ	Complexity Δ
src/Model/OrderStatusLog.php	30.43% <0%> (-19.57%)	16 <3> (+3)
src/Model/Order.php	57.5% <100%> (+0.17%)	60 <0> (ø)	:arrow_down:
src/Checkout/OrderEmailNotifier.php	69.81% <66.66%> (-0.19%)	21 <0> (+1)

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