This change adds `target` to the set of attributes allowed by the
HTML sanitizer which is applied to the description of a user_field.
The rationale for this change:
* If one puts a link (<a>...</a>) in the description of a user_field
that is present and/or required at sign-up, the expectation is that
a prospective new user will click on that link during sign-up.
* Without an appropriate `target` attribute on the link, the new page
will be loaded in the same window/tab as the sign-up form, but this
will obliterate any fields that the user had already filled-out on
the form. (E.g., hitting the back-button will return to an
empty form.)
* Such UX behavior is incredibly aggravating to new users.
This change allows an admin to add a `target` attribute to links, to
instruct the browser to open them in a different window/tab, leaving
a sign-up form intact.
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