add new level of diagnostic in system report

[diosmosis]

Currently there are three types of information in the system report: errors, warnings and everything else. Errors are highlighted red, warnings yellow. These are both noticeable and cause users to think they need to fix them (which may be correct, but may also be unnecessary). Everything else is meant to be informational and is unlikely to be actually read by the user.

There should be another level of information that is noticeable to the user (as in, it will be likely that they will see and read it), but does not make the user worry and think they have to immediately fix or follow up on it. Information that would fit this category would be, for example, the message displayed for users of Ninja Firewall. It is important that users of this plugin see this message to avoid spurious support requests, but they should not feel like something is wrong with Matomo.