In addition to the auto-suggesting widget, we want tag entry display a clickable list of available tags, which will be useful for users who aren't familiar with the project.
* When editing the tags, a floating box will appear below the message box asking the user to use official tags, with a link to expand.
* Clicking that link will open an overlay, contaiting the text box (which should remain in the same absolute position as the inline text box) and a three-column list of official tags.
* Tags which currently appear in the textbox appear highlighted.
* Clicking a highlighted tag unhighlights it and removes it from the textbox.
* Clicking an unhighlighted tag highlights it and adds it to the textbox.
* The list should get updated when the textbox is edited directly, so that as new tags are entered they get highlighted in the list (and vice versa).
de-targeting for now, since the current ui is probably good enough and we won't have time to do this anyway. we should consider this again after 3.0