The invitation message field of the wiki users administration is not linked to its label

Steps to reproduce

1. On a subwiki, go to Administration > Users.

Expected

The message textarea of the add or invite members form is labelled by its "Message" label.

Actual

WikiManager/WikiUsers.xml has <label for='message'> but the <textarea name='message'> has no id='message', so the label is not linked to the field.

The reviewer comment textareas of the candidates list on the same page have the same problem: their <label> elements have no for attribute.

Context

Found by the WCAG validation of SubWikiIT#joinOpenWiki, which reports the rule "Form elements must have labels".