Use modern javascript

[thatbudakguy]

Since we're bundling/transpiling anyway, it'd be nice to modernize our first-party javascript. That would involve:

• [ ] use let and const instead of var
• [ ] use ES6 imports/exports
• [ ] wrap things in classes where appropriate
• [ ] use template literals for string interpolation
• [ ] remove jQuery dependency where possible

[thatbudakguy]

Some work exists in #1479