Add confirm prop to ActionButton and ActionForm

[nhunzaker]

What

This PR adds a confirm prop to ActionButton and ActionForm that allows users to bail out of action dispatch if the result is false. Like:

function askUser(value, event) {
  return confirm("Are you sure you want to do this?")
}

function ConfirmButton(props) {
  return (
    <ActionButton action={deleteItem} confirm={askUser}>
      Delete Item
    </ActionButton>
  )
}

Why

ActionButton doesn't have a great way to confirm actions like deleting a record. For symmetry, I also added this functionality to ActionForm.

---

Note: This is against 12.x, our stable release. I'll eventually move this over to 13.x too.

[tommymarshall]

Very cool!

[codecov-io]

Codecov Report

Merging #521 into 12-stable will increase coverage by <.01%. The diff coverage is 100%.

@@              Coverage Diff              @@
##           12-stable     #521      +/-   ##
=============================================
+ Coverage      99.78%   99.78%   +<.01%     
=============================================
  Files             26       26              
  Lines            936      944       +8     
=============================================
+ Hits             934      942       +8     
  Misses             2        2

Impacted Files	Coverage Δ
src/addons/action-button.js	100% <100%> (ø)	:arrow_up:
src/addons/action-form.js	100% <100%> (ø)	:arrow_up:

---

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