Integrate the new addons flyout into the theme

[humitos]

We are working on a new addons flyout (https://github.com/readthedocs/addons) that emulates the old flyout shown at the bottom right for all the themes that are not Read the Docs Sphinx theme:

However, for our own theme, we are nicely integrating it on the bottom left together with the navbar:

So, we would use the integration points from the new addons to write <readthedocs-flyout></readthedocs-flyout> in the navbar of this theme (or using the JSON response) and display it integrated in the same way it currently is the old one.

• Trigger a CustomEvent on config load addons#60
• User API: communicate with the library addons#13
• Forwards compatibility (error handling) addons#17
• Backward compatibility addons#14

[humitos]

We have been testing this in our own documentation for months now and it has been working fine. I haven't had any negative feedback about it, so I think we should be ready to move forward with discussing what are the next steps here.