feat(Portal): use Ref for handling trigger refs

[layershifter]

Split from #1879. Also fixes problem with warnings about refs on stateless components and React 16.

Portal is the single component where we have handle refs on trigger prop as I see.

[codecov-io]

Codecov Report

Merging #2219 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #2219   +/-   ##
=======================================
  Coverage   99.73%   99.73%           
=======================================
  Files         151      151           
  Lines        2624     2624           
=======================================
  Hits         2617     2617           
  Misses          7        7

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

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[levithomason]

Portal is the single component where we have handle refs on trigger prop as I see.

Though this is true, I think we need to implement Ref anywhere we use this.refs on <ElementType /> as the user may pass a SFC in as also.

I will merge this one and we can do the others separately.

[levithomason]

Released in semantic-ui-react@0.76.0