Display sync trigger on sync page.

[JakePT]

Description of the Change

Displays a message indicating what triggered a sync on the sync page.

How to test the Change

• When syncing from the Sync page, the sync progress should read "Started manually from the Sync page"
• When syncing after saving feature settings, the sync progress should read "Started automatically after a change to feature settings"
• When syncing after installing, the sync progress should read "Started automatically after installing the ElasticPress plugin"
• The Sync History entries should have shorter versions of the same messages.

Changelog Entry

Added - The sync page now describes what triggered the current sync, and previous syncs.

Credits

Props @JakePT

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.