Fix for issue #398

[dmitryrogozhny]

Q	A
Bug fix?	[x]
New feature?	[ ]
New sample?	[ ]
Related issues?	fixes #398

What's in this Pull Request?

Issue: the Placeholder control is using global class names from Office UI Fabric. When no fabric.css file is available (I believe, this is the current default behavior in SharePoint Online), these classes do not work. Icon and text sizes will be smaller than expected in this case.

This PR changes global CSS class names with corresponding mixins, thus the Placeholder control does not depend on Office UI Fabric in runtime.

[msftclas]

All CLA requirements met.

[codecov-io]

Codecov Report

Merging #399 into dev will not change coverage.
The diff coverage is 50%.

@@           Coverage Diff           @@
##              dev     #399   +/-   ##
=======================================
  Coverage   73.31%   73.31%           
=======================================
  Files          18       18           
  Lines         802      802           
  Branches      164      164           
=======================================
  Hits          588      588           
  Misses        164      164           
  Partials       50       50

Impacted Files	Coverage Δ
src/controls/placeholder/PlaceholderComponent.tsx	96.66% <50%> (ø)	⬆️

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[dmitryrogozhny]

Modified failing tests for the Placeholder:

• changed selector for the Icon component as the i tag won't have ms-Icon--Add class (data-icon-name attribute is set instead).
• renamed duplicate test 'Test placeholder without button' to 'Test that placeholder renders' as the test only checks that component renders.
• disabled a warning message for the Icon component - WARN: 'The icon "Add" was used but not registered. See http://aka.ms/fabric-icon-usage for more information.'
• changed test condition for 'Test placeholder with custom classname' - it should have a top-level
  container with both standard and custom classes applied.

[AJIXuMuK]

Thank you @dmitryrogozhny for the fix!