added read-only evaluation based on space membership
added read-only evaluation based on space membership

initially set to true, read-only is disabled (i.e. editing enabled) if
the user is a member of the current space

NB: This evaluation happens asynchronously, leading to a brief "flash of
unmodifiable content".

FND/tiddlyspace,FND/tiddlyspace,TiddlySpace/tiddlyspace,TiddlySpace/tiddlyspace,FND/tiddlyspace,TiddlySpace/tiddlyspace