Add sticker support to our web-based chat
Many newer chat clients support "stickers", which are often a set group of photos or animations provided by the client that users can send in chat.
Add a new "stickers" button to our chat UI which opens a menu (with vertical scrollbar) for selecting and managing stickers. The first (and only original) "sticker" should be a rounded box with dashed border and a "+" in it for uploading a new sticker. When the "+" button is clicked a photo can be selected for upload (as we currently do with the photo button), but instead of sending a photo its added to the top of the sticker menu just to the right of the "+" button.
If a sticker (other than the "+" sticker) is selected, its sent as a thumbnail-only (not clickable)
<media> to the current chat tab. For the purposes of this task, stickers should only be stored for a single session (so reloading the page will reset the stickers menu) and no scaling will be done. Tasks for these and other features may be added after this task is complete.