Split out from WordPress/gutenberg#24760.

In the experimental Navigation screen, the 'Save navigation' button's text is not centred properly.

Currently, in order to register a field to an Edge you have to know the full Edge name, like RootQueryToPostConnectionEdge and that's cumbersome.

It would be nice to have something like: register_graphql_edge_field( $fromType, $toType, $fieldName, $config );

I think it's easier to know the from/to type than know the fully qualified edge name.

The list of AMP Caches is available at https://cdn.ampproject.org/caches.json

This could be pulled in dynamically rather than hard-coding each AMP Cache in \AMP_HTTP::get_amp_cache_hosts():

https://github.com/ampproject/amp-wp/blob/9e049f6554b69e06284ead57d13ef416226a51ec/includes/class-amp-http.php#L191-L239

This is low priority because the list of caches changes infrequently.

The buttons are stacking on top of each other.