Allowing styling for numerical and A-Z sort buttons to be passed into the FacetFieldPaginationComponent

[hudajkhan]

Closes #3491 .
What this PR does:

• Allows for passing the button-specific styling for the A-Z and numerical sort buttons as a parameter
• Sets the default classes for styling within the component so _facet_pagination does not need to add a parameter if the default styling is not to be overridden.
• Uses the styling within the html.erb file

P.S. This component didn't appear to have a direct rspec test before. If that makes sense to add, please let me know. Thanks

[sandbergja]

Looks great, thanks @hudajkhan !