Run SwiftLint when building the 'PushNotifications' build target

[danielrbrowne]

There was already a .swiftlint.yml file in the repo, and it was run as part of the Bitrise CI pipeline. However SwiftLint wasn't being run as part of the local build process.

This PR:

• Adds a SwiftLint build phase to the project file for the PushNotifications build target
• Autocorrects some SwiftLint warnings
• Manually resolves some other warnings
• Guards against accruing more warnings in the future

[codecov-io]

Codecov Report

Merging #164 (e752668) into master (2924188) will increase coverage by 0.73%. The diff coverage is 82.57%.

@@            Coverage Diff             @@
##           master     #164      +/-   ##
==========================================
+ Coverage   89.45%   90.18%   +0.73%     
==========================================
  Files          57       57              
  Lines        2912     2925      +13     
==========================================
+ Hits         2605     2638      +33     
+ Misses        307      287      -20     

Impacted Files	Coverage Δ
Sources/Persistence/Metadata.swift	100.00% <ø> (ø)
Sources/Persistence/PersistenceConstants.swift	100.00% <ø> (ø)
Sources/ReportEventType.swift	33.33% <ø> (-57.15%)	:arrow_down:
Tests/AuthDataTests.swift	100.00% <ø> (ø)
Tests/ConstantsTests.swift	100.00% <ø> (ø)
Tests/IntegrationTests/ApplicationStartTests.swift	100.00% <ø> (ø)
...IntegrationTests/MultipleInstanceSupportTest.swift	100.00% <ø> (ø)
Tests/Persistence/DeviceStateStoreTests.swift	100.00% <ø> (ø)
...ts/Persistence/InstanceDeviceStateStoreTests.swift	100.00% <ø> (ø)
Sources/API/NetworkService.swift	68.45% <14.28%> (-6.05%)	:arrow_down:
... and 25 more

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