Added custom excerpt to post revisions

[kevinansfield]

closes https://linear.app/tryghost/issue/MOM-170

When the subtitle field is included in the editor it creates a disconnect with post revisions if the underlying custom excerpt data is not included so we'd like to both preview and restore the subtitle when the in-editor subtitle field is enabled.

• added post_revisions.custom_excerpt column to schema
• added migration to add post_revisions.custom_excerpt to existing databases
• added migration to populate post_revisions.custom_excerpt with the current post.custom_excerpt value from the associated record
  • ensures no data is inadvertently lost when restoring an old version
  • using current data matches what would have happened previously where custom_excerpt was never overwritten when restoring an old version
• updated post revisions handling to accept the custom_excerpt field
• updated Admin's revision preview and restoration to display and set the custom_excerpt field

[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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.35%. Comparing base (9bf5a61) to head (4dcd3e3).
Report is 154 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #20323      +/-   ##
==========================================
+ Coverage   73.28%   73.35%   +0.06%     
==========================================
  Files        1264     1267       +3     
  Lines       74317    74735     +418     
  Branches     9886     9966      +80     
==========================================
+ Hits        54462    54820     +358     
- Misses      19056    19108      +52     
- Partials      799      807       +8     

Flag	Coverage Δ
admin-tests	42.53% <100.00%> (+0.37%)	⬆️
e2e-tests	81.85% <100.00%> (+0.03%)	⬆️

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