Deprecate html-related stuff

[Marcin Nowak]

There are many problems with creating something else than one-page-html-doc using Django. Please remove any html related stuff from framework, because Django is not a html framework but web framework (as mentioned in description of #18054), and "web" term is not reserved for HTML only.

• django.forms should be removed or HTML-based widgets should be cutted-off; forms should validate and clean data only
• technical error responses (404 and 500) should return responses dependent on Accept header value; there are many issues with debugging web services by digging HTML/JS/CSS output in shell/terminal
• remove html from django.utils.log
• remove html from django.utils.timesince
• if I forgot something - remove it too

[Tim Graham]

Thanks for the raising the issue, however, one monolithic ticket to address this doesn't seem the right approach to me. If you could open some smaller, actionable tickets focused on specific use cases where output needs to be customizable that would be better. I doubt we can simply deprecate or remove HTML in all of the areas your raise, but we could certainly have hooks to customize the output as needed.