Replace jshint with eslint

[simison]

Replace jshint with eslint. Just a quick stint, mostly deleted files and plugged some overrides to eslint config to see if this would be easy or complicated.

Complications come mostly from the fact that ES5 and ES6 files are mixed up in different folders, specifically _inc which has both.

Moving old JS files out from _inc would probably make this much easier.

Resolves #11914

Changes proposed in this Pull Request:

Added a ES5 eslint config file which is used for linting both modules/** and _inc/*(! not **) files, also fixed some linting errors in few files.
Might be a good idea to switch some of the rules from warn to error and fix these issues too.

TODO:

• Use migration ruleset from @wordpress/eslint-plugin
• Ensure eslint overrides paths are correct
• remove inline jshint rules (e.g.) - in followup PR
• Configure correct globals - in followup PR

Testing instructions:

yarn lint or yarn lint-es6 && yarn lint-es5

Proposed changelog entry for your changes:

• N/A

[matticbot]

Caution: This PR has changes that must be merged to WordPress.com
Hello simison! These changes need to be synced to WordPress.com - If you 're an a11n, please commandeer, review, and approve D26470-code before merging this PR. Thank you!

[jetpackbot]

Thank you for the great PR description!

When this PR is ready for review, please apply the [Status] Needs Review label. If you are an a11n, please have someone from your team review the code if possible. The Jetpack team will also review this PR and merge it to be included in the next Jetpack release.

Scheduled Jetpack release: April 29, 2019.
Scheduled code freeze: April 22, 2019

Generated by 🚫 dangerJS against 57cf0d3

[matticbot]

Caution: This PR has changes that must be merged to WordPress.com
Hello simison! These changes need to be synced to WordPress.com - If you 're an a11n, please commandeer, review, and approve D29957-code before merging this PR. Thank you!

[jetpackbot]

Thank you for the great PR description!

When this PR is ready for review, please apply the [Status] Needs Review label. If you are an a11n, please have someone from your team review the code if possible. The Jetpack team will also review this PR and merge it to be included in the next Jetpack release.

Scheduled Jetpack release: August 6, 2019.
Scheduled code freeze: July 30, 2019

Generated by 🚫 dangerJS against c1bd239

[matticbot]

simison, Your synced wpcom patch D29957-code has been updated.

[matticbot]

simison, Your synced wpcom patch D29957-code has been updated.

[matticbot]

simison, Your synced wpcom patch D29957-code has been updated.

[matticbot]

simison, Your synced wpcom patch D29957-code has been updated.

[matticbot]

simison, Your synced wpcom patch D29957-code has been updated.

[jeherve]

I like it. I think it would be nice to merge it as is, and then fix all the current es6 and es5 warnings in a follow-up PR, and then update the rules to match WordPress', so we don't add more warnings in the future:
https://github.com/WordPress/gutenberg/blob/4905299edfb1f652229daea695fe196b55d0bf76/packages/eslint-plugin/configs/jshint.js

This is going to need a rebase first though. I took care of that. Will merge when the tests pass.