Make "notes" more structured in the description of our mirror pool

So we’ve been using the “notes” field in our JSON schema for at least two different things very recurrently:

• URL of stats (8 times right now), so maybe having a dedicated “stats”
  string would be cool.

• Failures, with a date and a description. Most of the time we
  only keep the latest failure. Sometimes we keep two (see
  https://tails.unixcorn.org/tails/). So maybe this deserves a
  dedicated field as well. It could be a string called “disabled_because”.

Having these two out of the way would tidy up most of the things.

I want to try to implement this and learn a bit of JSON on the way.

Related issues

• Related to #14922 (closed)

Original created by @sajolida on 12127 (Redmine)