Improve ESLint config and React code

[philectron]

I originally opened this branch to remove the max-len rule from ESLint, but it seems master has some new stuff that are not in merge.

This branch was created from the latest version of master, so treat this PR as the update from master to merge. I recommend making PRs that target the merge branch only from now on.

CHANGELOG:

• Add a React plugin to ESLint
• Remove the max-len rule from ESLint
• Add Mergify config (I think the commits have diverged, despite both master and merge has the exact same file.)
• @clairecahill 's fix of PropTypes validation error by ESLint

[philectron]

@silverware13 Mergify should merge this PR into merge on one or more approvals, and GitHub itself should automatically delete the branch (per the "Merge Button" option of this repository). Keep an eye out to see if this automation works or not.

[philectron]

Good bot