Plugin web API onInstall dialog for onboarding users

[richardguerre]

Context

• Currently when the user installs a plugin, it shows a toast saying "Plugin installed". There is no indication what the plugin does or how it works.
• The user has to wonder around their Flow instance to see if something changed.

Solution

Plugins should be able to have users go through an onboarding experience after installing the plugin:

• Expose web.onInstall as an API for plugins to use.
• If the plugin returns null, then no onboarding experience is shown.
• If the plugin returns a component/JSX, then that content is shown within Dialog or Modal with a close button so the user know what to do.