Accessibility: Group radio buttons in fieldsets for Site creation

[mercime]

For radio buttons related to creating a new site in the "Create a Site" and the "Register" templates, some screen reader users who tab through form controls will be confused as to what the "Yes" and "No" buttons are all about. The text, "Privacy: I would like my site to appear in search engines, and in public listings around this network", is not associated to a form control and thus not read aloud by the screen reader.

To replicate: Close your eyes, open up a screen reader of choice, and tab through the form.

Note: in one instance, I think I heard only the word 'privacy' while tabbing through the form: ".. Site title (required) edit has auto complete, Privacy, Yes radio button checked 1 of 2, No radio button unchecked ..." but could not replicate it. But even if by chance they hear "Privacy", screen reader users have no idea what kind or level of privacy we are referring to for the new site.

To resolve this issue, we're surrounding the radio form controls for screens mentioned above with fieldsets and using the text for the legend. Ideally the legend should be shorter than the text we currently have because the legend will be read for each radio button. In this case though, clarity of information especially relating to privacy trumps the length.

[mercime]

In 11173:

Accessibility: Add fieldsets for radio buttons in Site privacy sections.

Fixes #7271.