Create a new ElasticPress.io page with client messages

[felipeelia]

Description of the Change

This PR introduces a message system between the EP.io infrastructure and EP.io customers, making it possible to warn users about maintenance windows, for example.

How to test the Change

Changelog Entry

Added - ElasticPress.io messages system.

Credits

Props @felipeelia @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.

[JakePT]

@felipeelia I have pushed a change to add a badge counter to the ElasticPress admin menu item, and added a transient to support this without having to query for messages on every page load. Still to be discussed is whether these need to be displayed on their own admin screen or if they can go in Index Health or similar.

[JakePT]

I’ve updated the PR. I have:

• Added an ElasticPressIo class for interacting with ElasticPress.io (a todo may be to move some of the Autosuggest and Instant Results template stuff into here).
• Move the method for retrieving endpoint messages to that class. I added code to cache the results in a transient with a 24 hour expiry. This was necessary for the count badge.
• Added a counter to the ElasticPress admin menu item, and Status Report submenu item, indicating the number of messages found with the above method.
• Added support for 'messages' to Status Report reports. The ElasticPress.io report gets messages using the above method.
• Updated the Status Report UI to print messages as notices under each report heading.
• Moved the ElasticPress.io report, and the Failed Queries report, to the top of the Status Report.

[anjulahettige]

Ping ElasticPress every 1 hour.