Set stage size initialState to small

This change means that Scratch starts with the stage (where users can
run their code) in the "small" mode. They can then toggle to
"large" or "fullscreen". The reason for this change is to make the
scratch interface we embed in Experience CS more suitable for narrower
screens, where we still need to leave room for the instruction panel
next to Scratch.

I considered making a more extensive modification to gui.jsx[1] where it
calls `resolveStageSize` to take into account the size of the
containing element, rather than the size of the screen/browser
window (I think this code will have been written with the assumption
that Scratch is taking over the whole viewport).

In the end I decided against this - it'd be a more invasive change and
one that would probably require quite a bit more testing and
longer-term maintainance in case of upstream changes. Given that the
user has the option to toggle to a larger stage themselves, I think
for now this very small change to the default is preferable.

[1] https://github.com/RaspberryPiFoundation/scratch-gui/blob/a226c7a6ed58bc4b3b25bbb9bce40db5d328b7d1/src/components/gui/gui.jsx#L147

Author: chrislo

src/reducers/stage-size.js

@@ -3,7 +3,7 @@ import {STAGE_DISPLAY_SIZES} from '../lib/layout-constants.js';
 const SET_STAGE_SIZE = 'scratch-gui/StageSize/SET_STAGE_SIZE';
 
 const initialState = {
-    stageSize: STAGE_DISPLAY_SIZES.large
+    stageSize: STAGE_DISPLAY_SIZES.small
 };
 
 const reducer = function (state, action) {