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
b447a26832