fixed saving of automatically generated ColorPalette
fixed saving of automatically generated ColorPalette

If a ColorPalette tiddler already existed in the given space (due to
inclusion), its fields were erroneously used for saving the new
ColorPalette, leading to a 403 due to the wrong bag being targeted.

Accordingly, RandomColorPalette's generatePalette has been refactored to
return the new palette as a string rather than automatically saving a
tiddler to the store.

NB: Also added exclude* tags for consistency with custom shadow tiddlers
like StyleSheet, MarkupPreHead etc.

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