Create starter blueprints install link

[mnholtz]

Part of Onboarding Spring 4

We would like a way to "recover" from the situation where a user might find themselves on the Playground page, but have Playground blueprints missing. As a solution, we'd like to add a link to the extension that will install Playground (starter) blueprints if they aren't already installed, and navigate back to the Playground page.

This way, we can 1) modify the Playground landing page to include this url if the user really is missing the required blueprints and 2) invite users to try the Playground in other situations rather than just a first-time install (e.g. invite existing users to try out the Playground).

Implementation sketch

1. Broaden starter blueprints api URL to return starter blueprints regardless of starter blueprint install flag (back-end) https://github.com/pixiebrix/pixiebrix-app/issues/2247
2. Expose a starter blueprint "should install?" url (back-end) https://github.com/pixiebrix/pixiebrix-app/issues/2247
3. Create a route on the options page that
   a. Calls the starter blueprint url and gets starter blueprints
   b. Installs them if they aren't already installed with a friendly loading message/spinner
   c. Opens/redirects to the playground page afterwards
   d. Modify the existing background script that installs starter blueprints to use back-end api changes to starter blueprints