common click-outside JS helper

We often want to run some code when clicking outside of an element. examples:

• sidebar: clicking outside on closes it (on small screens)
• modal: clicking outside should close it
• dropdowns: clicking outside should close, possibly save value/submit form

We could do one of:

• util function, takes an element (or selector) and a callback
• data attribute on elements
• Vue wrapper component
• Vue mixin (example)

---

Also useful is 'excluded class' for things you can click on that won't be considered outside click. Shown in this Ember lib