Restrict Content: Add caching warning notice

[n7studios]

Summary

Adds a notice to the Member Content settings screen at Settings > ConvertKit > Member Content (beta), advising that any server side or plugin level caching system must be configured to disable caching when the ck_subscriber_id cookie is present.

Whilst we forcibly exclude caching by appending a query parameter as part of the authentication flow, two scenarios still exist where cached (incorrect) content can be served by the server or a third party caching plugin:

1. If a subscriber subsequently leaves the WordPress Page, and then accesses it days later (e.g. through a link on the web site, perhaps in the site navigation menu, which will not include a query parameter)
2. A different visitor accesses the same page; some caching plugins may take the member content output from an authenticated subscriber, cache it, and therefore serve it to all visitors.

Most third party caching plugins provide a setting to disable caching when a cookie is present - for example, but not limited to:

W3 Total Cache

WP Fastest Cache

WP Super Cache

It may be possible to automatically add this exclusion rule to third party caching plugins from the ConvertKit Plugin, but given the range of caching solutions available, we can't cover every single one, and this would involve some technical work - so the notice is a good starting point.

Testing

Existing tests pass.

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)

[noelherrick]

Is this missing a word? Aren't all plugins server side?

[n7studios]

Reworded for clarity to If your web host has caching configured (or you are using a caching plugin). Caching might not be performed by a plugin (for example, this ticket, where SiteGround have caching enabled in their hosting configuration).