Show a single sync option for the initial sync.

[JakePT]

Description of the Change

If a user skips setup when they visit the Sync page they are presented the usual Sync and Delete & Sync options. Th problem is that a Delete & Sync is required for the first sync, so if the user just presses Sync errors will occur.

This PR changes the Sync page so that only the correct syncing option is displayed when a sync hasn't been performed yet.

For implementation this PR:

1. Abstracts the sync panels into a SyncPanel component to reduce repetitive code.
2. Hides the Delete & Sync panel and changes the wording of the Sync panel if there is no information for a previous sync.
3. Forces put_mapping for any Sync requests from the Sync page if there has not been a previous Sync.

Closes #2695

Alternate Designs

I've opted for changing the behaviour and wording of the top Sync option for the first sync, rather than just displaying the "Delete & Sync" option. This is because I though it could be confusing if the user was asked to "Delete & Sync" when there was nothing to delete.

For the wording of the panel I drew on the wording of the Getting Started page of the website, and the documentation, but this could be anything.

Possible Drawbacks

I'm not aware of any.

Verification Process

After entering the hosting details during install choose to Skip Setup to bypass the initial sync._ Then manually navigate to ElasticPress > Sync. Instead of seeing both Sync and Delete & Sync panels, only a single panel should be displayed.

Similarly, when choosing to Sync during setup, this is the only panel that should appear during that sync.

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

Changelog Entry

Fixed - An issue where syncing after skipping setup, instead of deleting and syncing, resulted in an error.

Credits

Props @JakePT