Comments Form: Allow Hiding Website Field

#### Is your feature request related to a problem? Please describe.

A lot of readers today won't have a website, and so displaying a "Website" field can provide some confusion; I often get people entering my own website link into the field, especially as there's no indication that it's optional. It would be preferential if an option could be provided to hide it, since it's never really been useful for me and is just causing confusion.

#### Describe the solution you'd like

Since the form is an iframe, I think that changes will need to be made outside the Jetpack plugin.

https://github.com/Automattic/jetpack/blob/98d6422c8504d1e22e0f1a52c816ea7f5da36336/modules/comments/comments.php#L337

Ideally, an option or filter to hide the Website field could be provided. Alternatively, the ability to add emphasis that fields are optional may alleviate confusion.

#### Describe alternatives you've considered

If I recall correct, before my site became an Atomic one, I was able to use some styling to tweak the form since there was no iframe. As there is now, the alternatives are limited.

#### Additional context

Note that it's unclear that the field is optional, leading to a lot of commenters entering my own site's URL. It'd be much easier to hide it.

Theme in screenshot: Ashe

Start with the comments form entry point linked in modules/comments/comments.php around line 337, then trace how the iframe form is assembled outside the plugin. The work is complete when site owners have a documented option or filter to hide the Website field, with the resulting comments form behavior verified.

Written by the indexing model from the issue text.