Fixed #23913 -- Deprecation of `=` comparison in `if` template tag

This is a pull request for ticket [#23913](https://code.djangoproject.com/ticket/23913) that deprecates the "feature" that allows for comparison in `if` template tag using `=` operator (instead of `==`). There was some dicussion in the tickets if this change should be made, but I believe that we shouldn't let people create wrong coding patterns, even if it means that one day they will have to fix mistakes of using this undocumented feature in their old code.

I am not entirely sure that this is how you do deprecation path and I also couldn't find docs for that in contribution guidelines, so I'd be happy to fix whatever I did wrong :)