Move admin_post_pantheon_cache_delete_page from WP upstream

[ari-gold]

Let's update the button text from "Delete Cache" to "Clear 's Cache" so e.g:

"Clear Post's Cache"
or
"Clear Page's Cache"

[danielbachhuber]

@ari-gold The upstream included this functionality across all URIs, so I decided to replicate that instead of making it specific to posts / pages.

Here's the button:

When the page cache is purged successfully, the text changes slightly to provide visual confirmation:

[ari-gold]

Nice! I think "Clear Page Cache" still isn't obvious that the button clears the cache for the current URL. Let's go with "Clear URL Cache"

[danielbachhuber]

Let's go with "Clear URL Cache"

👍

[ari-gold]

Looks good. Thanks @danielbachhuber!