Update SwiftLint configuration in the project and run it on CI

[iangmaia]

Description

This PR sets up SwiftLint as CocoaPods dependency and runs it on CI using the command run-swiftlint.

The main changes:

• .swiftlint.yml file update and definition of the SwiftLint version to use.
• Use of run-swiftlint command from a8c-ci-toolkit-buildkite-plugin in a new Buildkite step.
• Configure SwiftLint via CocoaPods.
• Configure project to run SwiftLint as a Build Phase.
• Fix the currently found violations in the codebase.
• Remove the SwiftLint Rake and Hound configurations.

Testing Details

Make sure CI is 🟢

[dangermattic]

	3 Warnings
⚠️	View files have been modified, but no screenshot or video is included in the pull request. Consider adding some for clarity.
⚠️	This PR is larger than 500 lines of changes. Please consider splitting it into smaller PRs for easier and faster reviews.
⚠️	PR is not assigned to a milestone.

Generated by 🚫 Danger

[wpmobilebot]

You can test the changes in simplenote-ios from this Pull Request by:

• Clicking here or scanning the QR code below to access App Center
• Then installing the build number pr1554-f63e68a-018de6d8-fa8d-440b-a3d8-399e9c190964 on your iPhone

If you need access to App Center, please ask a maintainer to add you.

[mokagio]

The UI tests failure is a long standing issue and has nothing to do with these changes.