Add "purge edge cache" to EmDash admin portal

[kodster28]

When using an edge cache as part of EmDash -- such as the Workers' Cache -- there's a cross-team gap if any issues come up.

The edge cache settings are usually controlled by a different group of folks from the content publishers and -- as such -- require time sensitive escalations if there are any caching issues. Adds delay, confusion, etc.

Would be ideal to have purge cache button available within the admin settings of the dashboard, so admins can trigger their own full purge of the cache. Still want limited access here -- b/c this isn't a trivial event -- so ideal to be gated behind Admin perms.

[MA2153]

Sounds good to me. We are assuming EmDash purges content automatically upon update and this is a fallback mechanism in case something goes wrong.

[ascorbic]

I think a button to purge the cache for a single entry would be confusing, and would lead to a lot of people thinking it needs to be done manually. A button inside settings that allows manual purge of the whole cache might be a good idea though. Also one that lets you manually specify a path to purge.

[MA2153]

I assume the proposal is for purging the whole cache as a last measure when something goes wrong. I don't see anything that implies it's per entry.