breaking(Input): remove shorthand action button class

[levithomason]

Breaking Change

Dropdown actions must now explicitly define the button prop.

Before

<Input action={<Dropdown />} />

After

<Input action={<Dropdown button />} />

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Fixes #1703. For more context, see #1703 (comment).

[codecov-io]

Codecov Report

Merging #1791 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #1791   +/-   ##
=======================================
  Coverage   99.75%   99.75%           
=======================================
  Files         142      142           
  Lines        2441     2441           
=======================================
  Hits         2435     2435           
  Misses          6        6

Impacted Files	Coverage Δ
src/elements/Input/Input.js	100% <100%> (ø)	⬆️

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