Add a class to comment_form's <form> output

[chrisvanpatten]

It's a tad frustrating that you can't add a class to the <form> tag that's generated in the comment_form function. I know you can wrap the function in a <div> but it is gratuitous markup. Bootstrap (and I'm sure other frameworks) suggest adding classes to <form> elements too, and while they probably work by wrapping with a <div> as well, it is frustrating you can't follow their recommendations exactly.

There have been a number of tickets about the ability to add classes to the comment form's submit button—#20446, #15015, et al.—but none (that I can find) focusing on the ability to add custom classes to the <form> tag itself.

Thanks!

[chrisvanpatten]

This is my first patch, so I hope I did it right!

[SergeyBiryukov]

Duplicate of #23851.