Renamed subtitle to excerpt

[kevinansfield]

no issue

We've settled on using "excerpt" naming in place of "subtitle" to better reflect the underlying property name and tie in with themes and historical usage.

• added migration to rename the show_subtitle newsletter setting to show_excerpt
• renamed all places in the codebase that referenced subtitle

[github-actions]

It looks like this PR contains a migration 👀
Here's the checklist for reviewing migrations:

General requirements

• Satisfies idempotency requirement (both up() and down())
• Does not reference models
• Filename is in the correct format (and correctly ordered)
• Targets the next minor version
• All code paths have appropriate log messages
• Uses the correct utils
• Contains a minimal changeset
• Does not mix DDL/DML operations

Schema changes

• Both schema change and related migration have been implemented
• For index changes: has been performance tested for large tables
• For new tables/columns: fields use the appropriate predefined field lengths
• For new tables/columns: field names follow the appropriate conventions
• Does not drop a non-alpha table outside of a major version

Data changes

• Mass updates/inserts are batched appropriately
• Does not loop over large tables/datasets
• Defends against missing or invalid data
• For settings updates: follows the appropriate guidelines