Refactor occurrence picker toggle. Disable when not editing dates, props @mparisot, closes #35.
Fixes issue with venue combo box not working with no input. eo_get_venue_description() is not needed - and shouldn't be used in admin.
JS tidy up

Signed-off-by: Stephen Harris <contact@stephenharris.info>
Don't check permissions here - function is called by EO automatically. Fixes authors can't create events bug.
JS tidy up