Enhancement: show a notice and link to clear the cache when the theme is changed

### Impacted plugin

Super Cache

### What

The plugin should show an admin notice and a "clear cache" button when the theme is changed

### How

As mentioned in this post, WP Super Cache does not clear the cache when the theme is changed. I believe this is the right thing to do because the theme may be changed to test a new one temporarily.
However, an admin notice containing a "Clear Cache" button on the themes page that appears when the theme is changed would be a nice shortcut.

If caching is enabled for logged-in users, we should display an admin notice on the themes page informing the user that the cache must be cleared manually.