Add CI instructions to test generated HTMLs

• Can test not only external URLs, but also internal ones, as well as alt texts and other stuff. Individual HTML tags can be ignored
• Run in .post stage (for now) since we don't need it to halt anything with its result
• .htmltest/refcache.json file can be used by any job running htmltest, so it's cached with the htmltest-refcache key
• Test results are saved as an artifact