drafts: Use span instead of em for placeholders without DM recipient.

The `em` tag is typically used to denote emphasis, which may not be
semantically appropriate for placeholder text, where we just want to
style the text as italic.

web/templates/draft.hbs

@@ -34,7 +34,7 @@
                     {{#if has_recipient_data}}
                     <span class="private_message_header_name">{{t "You and {recipients}" }}</span>
                     {{else}}
-                    <em>{{t "No DM recipients" }}</em>
+                    <span class="drafts-unknown-msg-header-field">{{t "No DM recipients" }}</span>
                     {{/if}}
                     {{/if}}
                 </div>