add FontAwesome icon support to Button component and only make pluggable ui buttons center if they don't set the alignment

[s-cork]

this handles the case when a sign in button gets a font awesome icon from the users service

[s-cork]

This change is part of the following stack:

• add FontAwesome icon support to Button component and only make pluggable ui buttons center if they don't set the alignment #335 ◀

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