fix(button): Increase specifity of button icon's CSS class

[bwobrien]

Ensures that overrides apply over the styles defined by .material-icons.

[codecov-io]

Codecov Report

Merging #2242 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2242   +/-   ##
=======================================
  Coverage   99.13%   99.13%           
=======================================
  Files          90       90           
  Lines        3830     3830           
  Branches      496      496           
=======================================
  Hits         3797     3797           
  Misses         33       33

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