Plugins: include search results from the plugin browser when searching installed plugins

[v18]

Reported by @rickybanister: Currently if you try to use the search at /plugins it only filters your installed plugins. Also the only access to the plugin browser is from the tiny add button in the sidebar. There's an opportunity here to include results in a special block in the search results. Instead of showing a no-results message we could instead include a block of plugin results that the user does not yet have installed.

Here's a quick and dirty mockup:

cc @MichaelArestad for having the idea

This is related to #829 - better display for search results when there are none matching

[stale]

This issue has been marked as stale because it hasn't been updated in a while. It will be closed in a week. If you would like it to remain open, can you please comment below and see what you can do to get things moving with this issue? Thanks! 🙏