Provide more statistics

For example:

• Number of linked instances
• Total number of posts/comments/users/communities (This way we can calculate which percentage of the whole Lemmyverse is seen by a given instance)
• Percentage of local posts/comments which are tagged for each language. So we can reliably find out the primary language for an instance.
• user retention (percentage of users that are still active after 6 months? 1 month?)
• ban rate
• percentage of accepted signups vs failed/declined (maybe this just promotes accepting spammers)

Related discussion

These can be provided as part of /api/v4/site, or a separate /api/v4/stats endpoint. Instead of a scheduled task which stores the numbers in the db, it should be suffiicient to query these numbers directly in the api handler, and cache the result in memory.