Fullscreen button for the "submit new dweet" area

Something I missed a few times when working on a new dweet is the ability to see how it behaves in fullscreen. This is especially true for interactive tweets, where tracking the mouse coordinate well might be a bit annoying in fullscreen currently. Would be great (and make sense) to have the "fullscreen" option for the "working area" too.

Start by locating the submit-new-dweet working area and the existing fullscreen behavior in the JavaScript frontend. Done means the working area offers a fullscreen option and interactive dweets behave correctly while fullscreen, including mouse-coordinate tracking.

Written by the indexing model from the issue text.