tagify: working around contenteditable cursor

In order to correctly position the cursor in a contenteditable element, the
browsers display the cursor only when there is an element to properly flow
inside the contenteditable.

For Firefox, this is a `<br type="_moz">`. This `<br>` sticks to the end of the
contenteditable and does not disappear. For webkits, this is likewise a `<br>`,
but as you start typing, the `<br>` will become replaced with the TextNode
containing your text content. We then rely on input event to check to see if
we need to manually inject a `<br>`.

Finally upon creation of a tag, we focus on the `<br>` so as to position the
cursor correctly.

- [x] @attamusc