[UX] Dismissible status report messages.

Description of the need

In some cases, there will be errors or warnings on the status report that administrators will choose to accept, and ignore. It would be nice to be able to mark these messages as dismissed, so that they do not 1) trigger the red-dot if they are errors, or 2) clutter the status report.

Proposed solution

I would like every error or warning on the status report to provide an X that administrators could use to dismiss the message.

When dismissed, the message would be moved to one of the following places (discuss!) * a collapsed fieldset at the bottom of the status report * it's usual position in the list of status report messages - but grayed out as though disabled

When dismissed, the X on that message would be replaced with another tool that would provide the ability to re-enable the message. (suggestions?)

Additional information

This feature was recommended by @quicksetch in #3824 and would also help progress other issues, like #3490 and #5106.

Other things we discussed being dismissible: * dismissible backdrop_set_message()s * dismissible dashboard blocks,