Add HTML validation test in CI

[SaptakS]

Carrying it here from the discussion mentioned in #885 (comment)

https://github.com/svenkreiss/html5validator/ is the current python tool that can be used to do HTML validation. Though it seems to work on files and links, and not HTML string. As suggested by @chigby I am going to take a look to see if using django testing, we can pass the response.content directly to the html5validator's class to get some form of desired output (or something else that makes more sense).