Move front end editor modifications out of `functions.php`

[StevenDufresne]

We add some inline scripts to control Gutenberg behavior on load.

It's probably a better strategy to move that out from the functions.php file and into a separate javascript file because I expect we will need to add more logic of this type and we can keep it cleaner by just having it include the file.

Examples:

• Turning off the welcome guide. (Ref)
• Adding middleware to patch API call (Ref) (Ref) (Ref).