Prevent or alert missing Gutenberg plugin

[tellyworth]

The new themes are designed to work with the Gutenberg plugin installed, so we can make use of new features and fixes.

This means that if the plugin is disabled, and the site reverts to core's bundled editor, some features or layout might change.

To avoid this, the parent theme(?) should either prevent the Gutenberg plugin from being disabled, or at least alert if it gets disabled. It could produce a dashboard notice and/or ping the dotorg-alerts channel.

Context: https://wordpress.slack.com/archives/C02S78ZAL/p1662983457375559

This regression happened because the plugin was inadvertently disabled.

[iandunn]

WordCamp.org has an mu-plugin to do this, which could be easily modified for w.org.

It might be better to place it in wporg-mu-plugins, since there're likely other plugins we want to remain active that are unrelated to this theme (e.g., Stream).