Addons: split settings page per-addon

[humitos]

In the short term, as we keep adding configuration options for each addon, we will need to split the addons settings' page per-addon somehow. I'm creating this issue here to keep track of this and have this discussion together.

There are some different proposals here:

• Use the same settings page for all the addons and add a header for each of them (idea partially implemented in Addons: update form to show all the options readthedocs.org#11031)
• Split the settings page into different pages per-addon (different URL, forms, etc)
• Split the settings page into different tags per-addon (same URL, form, etc)

Reference: readthedocs/readthedocs.org#11069 (review)