Remove Cache purge form

The Fastly cache purge page (admin/config/services/fastly/purge) provides a user interface for purging caches.

However, there needs to be clearer descriptions about how to use the form, and some other improvements.

• Purge by URL: should actually be the path (and not the full url). We should probably strip out the full url. Or even better, allow both (in case users want to purge caches from another domain).
• To purge the homepage, they need to use /.
• We should also allow <front> to be used.
• Describe that surrogate keys are now based on url segments (since #2316577: Surrogate-Key integration based on URL segments → )
• Purge All is dangerous (this is already documented). I think we should have another permission for this, or an option to disable it completely.