Build a content type that allows a searchable A to Z list

There are A-Z lists on the site that are built manually but these create usability problems as items are not cross referenced in all useful ways. This feature will add a content type that lets authors add items to an A-Z list and specify alternate terms that items should be referenced under. It will be searchable and filterable for visitors.

Example pages where it would be used:
https://www.mass.gov/state-a-to-z Notice that the Governor's office is under "O" and not "G". This why things need to be cross posted.
https://www.mass.gov/guides/a-to-z-quick-links-massdep

Design: https://projects.invisionapp.com/share/EXFYA218JWS#/screens/277735729_A_To_Z_Sticky

**Reference**
JIRA DP-9453

Review the two example A-Z pages, the linked InVision design, and JIRA DP-9453 first; the issue names no repository files or tests. Define the content type around alternate terms, searchable and filterable visitor views, and cross-referenced entries, then verify it against the stated examples and design.

Written by the indexing model from the issue text.