Proof-of-concept emoticon support

This adds support for "emoticons" per #3.

A script replaces (for example) `:-)` in message text with `<span class="subrosa-image-emoticon" title=":-)"></span>`.

Currently six variants of the 'smile' emoticon are defined (but all are styled the same). To add more, put images in [./img/emoticons](./img/emoticons), add text aliases in [./js/app-emoticons.js](./js/app-emoticons.js) and add styles in [./css/image-emoticons.css](./css/image-emoticons.css).