Add digest category to email notifications settings

[mattsherman]

This PR adds the digest category to the email notification settings screen in /me/notifications/updates.

In addition, the existing file was refactored to remove all eslint warnings except for the CSS classname warning -- getting rid of that warning would require more extensive renaming/moving of directories in client/me/notifications-settings, so that is not being done at this time, in order to keep this PR small and focused.

To test this PR:

• Check and uncheck categories and make sure things are saved/loaded properly and function as before

[matticbot]

Test live: https://calypso.live/?branch=add/digest-mailing-list-to-settings

[mjuhasz]

Code changes look good. Thank you for the clean-up, you're a good Boy Scout 👍

Tested all categories on calypso.live, worked as expected, no errors or warnings in the console.

Approved!