Hide unlisted plugins entirely on the frontend

The /data/plugins_listing endpoint returns a map from known plugin
names to whether each plugin is currently active (true or false).
The frontend treats absent keys in this map the same as if the key were
set to false. Thus, if the frontend knows about a plugin that the
backend does not, then the plugin will be marked as inactive.

It would be convenient if missing plugins were instead hidden from the
UI entirely. This would enable us to easily gate new, experimental
plugins behind a feature flag (--alpha_plugins) without having to
build distinct copies of the index.html page. The backend could simply
omit unstable plugins from its response, and the frontend would treat
them as if they didn’t exist at all.

cc @erzel

Start by locating the frontend code that consumes the /data/plugins_listing response and the plugin entries rendered in index.html. Trace how absent keys are currently interpreted, then verify that plugins omitted by the backend are absent from the UI while explicitly inactive plugins remain handled as before.