Add edtf-humanize gem for better date display

[wickr]

Descriptive summary

With all of the work being done to clean up dates and get to valid EDTF, and validation already added, we can now use the edtf-humanize gem to better display the date values. This is especially helpful for decades, centuries and other intervals.

https://github.com/duke-libraries/edtf-humanize

Expected behavior

Dates on show page and related pages should display using this gem. Edit form should still show EDTF value in the form. Unparseable dates should fail gracefully, still showing the raw value, and not break further page rendering.

Related work

#40 #1097 #1708

Accessibility Concerns

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[CGillen]

Related to #2199