Integrations: add more integration metadata

[agjohnson]

In #617 I left some notes for next additions to the integration listing, for both GHA integration and the other existing integrations:

• GHA integration could show "{number} exchanges" as well if there is a way we can get this data.
• Or if we can't do that, "Last update 10 minutes ago" would be great still. This seems more likely.
• With the GHA integration showing disconnection status, the other integrations could show a disconnected state too. We'd need a way to figure out how to actually detect that first though.
• We should consider storing the GHA installation data on the Integration model and using that for the GHA edit/config URL. This is so we can link to the GHA installation previously used if someone removes the repository permission from the installation. ¹

Footnotes

1. Noted here https://github.com/readthedocs/readthedocs.org/blob/fc41cd2fb2f3ad4696d7bd3469614288466f1f80/readthedocs/integrations/models.py#L370-L374 ↩