Monitor servers from the htpdate pools

Originally created by @bertagaz on #11562 (Redmine)

While tackling tails/tails#10494, it came up that some of the HTTP servers of the htpdate pools were buggy. This has some incidence for Tails to boot correctly, and our test suite to run nicely. We should monitor if this servers are up and answering correctly to the CURL requests made by htpdate to ensure this service is reliable.

Note: For S11, this fits in:

• D.1 - Make Tails development process more efficient: Improving developers' visibility on possible trends related to servers in the htpdate pool allows them to more easily assess and improve the boot UX of the next release.

From FT's perspective

• Not super important to me now that we have Gitlab CI doing a part of the job. However, that GitLab CI check doesn’t do it all (ie: no Tor in the loop) nor perfectly (no flap detection).
• This impacts users, not just developers.
• We only need email notifications to FT, no access to web interface. FT would be responsible for handling failures.