Monitor servers from the htpdate pools

While tackling #10494 (closed), 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.

Related issues

Related to #13472 (closed)
Related to #10494 (closed)
Related to #17233
Related to #12023 (closed)
Blocks #10495
Blocks #13242

Original created by @bertagaz on 11562 (Redmine)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information