webmail: add button to create a mailbox below another one

before this, you could use the button at the top of the list of mailboxes to
create a submailbox somewhere, and you would have to specify the full path of
the new mailbox name. now you can just open up your Lists/.../ mailbox, and
create a mailbox below that hierarchy.

webmail/webmail.ts

@@ -5156,6 +5156,32 @@ const newMailboxView = (xmb: api.Mailbox, mailboxlistView: MailboxlistView, othe
 						await withStatus('Marking mailboxes as read', client.MailboxesMarkRead(mailboxIDs))
 					}),
 				),
+				dom.div(
+					dom.clickbutton('Create mailbox', attr.title('Create new mailbox within this mailbox.'), function click(e: MouseEvent) {
+						let fieldset: HTMLFieldSetElement
+						let name: HTMLInputElement
+						const ref = e.target! as HTMLElement
+						const removeCreate = popover(ref, {},
+							dom.form(
+								async function submit(e: SubmitEvent) {
+									e.preventDefault()
+									await withStatus('Creating mailbox', client.MailboxCreate(mbv.mailbox.Name + '/' + name.value), fieldset)
+									removeCreate()
+								},
+								fieldset=dom.fieldset(
+									dom.label(
+										'Name ',
+										name=dom.input(attr.required('yes')),
+									),
+									' ',
+									dom.submitbutton('Create'),
+								),
+							),
+						)
+						remove()
+						name.focus()
+					}),
+				),
 				dom.div(
 					dom.clickbutton('Move to trash', attr.title('Move mailbox, its messages and its mailboxes to the trash.'), async function click() {
 						if (!trashmb) {