Prompt admin if cache should be wiped after activating/deactivating a plugin

[raamdev]

After a plugin is activated or deactivated, Comet Cache should prompt the administrator (via a Dashboard notice) if the cache should be wiped, along with some basic recommendations for site owners who may not be sure what to do.

If a plugin is installed that affects the front-end of the site, those changes may not show until the cache has been cleared, so it would be a good idea to make the default to wipe the cache, with the admin prompt optional for more advanced site owners who may want to skip the wipe.

This issue depends on Issue #307 (Option to prompt admin before performing site-wide wipe of the cache) being implemented first.

This should be a Pro feature.

Reported via #170.

[raamdev]

Note: As of Quick Cache v150129, installing, activating, deactivating, or deleting a plugin does not wipe/clear the cache. This is unexpected behavior and should be fixed before this feature is implemented. See #424

Blocking issue: #424

[raamdev]

The blocking issue #424 has been closed as of v151220 and this issue is now ready for work.

[raamdev]

Related: #374