Compatibility layer - Add manual fix required warning for WP Cerber

[jazzsequence]

WP Cerber's anti-bot-spam features make the site uncacheable.

This PR adds WP Cerber to the compatibility layer to flag as an issue on the site health status page.

Automatically fixing the issue was investigated but discarded when setting the apparent options in the database did not appear to resolve the issue.

This PR also adds a filter that allows the list of known incompatible plugins to be modified (e.g. if you know there's an issue and want to remove the warning from the Site Health page) and documentation for the filter.

Fixes #52

Partially blocked by pantheon-systems/documentation#9164 (we need the documentation to exist before this should be included in a release)

[pwtyler]

This LGTM. I would love to see the site-heath array DRYed up a bit, down to an array of unique values/types if, for example, the plugin message is going to be the same link with a different achor, or the plugin status is one of only a couple values.

[jazzsequence]

@pwtyler I agree. As I dig into it more, I find places to improve things. I think we can just continue to make it better over time.