Make custom icon directory name configurable via theme options #1680

Job	Run time
trigger-edge-build	0s
jest (pageflow, package)	1m 51s
rspec (pageflow, 3.2, ., bin/rspec --tag ~js, true)	5m 48s
rspec (pageflow, 3.2, ., bin/rspec-with-retry --tag js)	4m 34s
jest (pageflow-paged, entry_types/paged/packages/pageflow-paged)	1m 42s
rspec (pageflow_paged, 3.2, entry_types/paged, pageflow_paged, bin/rspec)	4m 30s
jest (pageflow-paged-react, entry_types/paged/packages/pageflow-paged-react)	1m 53s
rspec (pageflow_scrolled, 3.2, entry_types/scrolled, pageflow_scrolled, bin/rspec --tag ~js)	4m 42s
jest (pageflow-scrolled, entry_types/scrolled/package)	2m 32s
rspec (pageflow_scrolled, 3.2, entry_types/scrolled, pageflow_scrolled, bin/rspec-with-retry-on-t...	4m 22s
	31m 54s

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