Refactor Works.PubDate and Works.PubDateSource

[mattieburkert]

To do:

Refactor Works.PubDate as Works.Date and Works.PubDateSource as Works.DateSource

Reason for change:

Now that we are hooking up related witnesses to dramatic works, it's more accurate to refer to witnesses (not works) having publication dates. In some cases, the work date (not witness date) is an approximate or actual date of first performance, not publication. This is particularly true with pre-Civil War plays for which dates have been newly added using the DEEP Playbooks database (https://deepplaybooks.org/).

Scope of change:

• three .php files
• Sphinx index
• documentation

The change to the Sphinx index should be tested rigorously to ensure there are no deprecated references anywhere in Sphinx-related functions or configuration files, as these can cause the index to fail to build.