Option to prompt admin before performing site-wide wipe of the cache

[raamdev]

When Quick Cache detects that the entire cache needs to be wiped, it would be nice to have an option to force manual approval of that event by an admin (via a Dashboard notification). That way, on large sites, a site administrator who understands the side-effects of leaving the cache stale, can make an education choice and decide if they want the entire cache wiped or if they want to delay that action.

This would be very helpful on large sites who may have a huge cache that took some time to build and which they would prefer to only wipe at certain times.

Such an option should, of course, be off by default, as many site owners won't realize the negative impact delaying a wipe might have.

This should be a Pro feature.

Requested via #170.