This reduces chances of errors where consumers of strings mutate inputs and reduces memory usage of the app. Test suite passes now, but there may be some stuff left, so we will run a few sites on a branch prior to merging
* FEATURE: backend support for user-selectable components * fix problems with previewing default theme * rename preview_key => preview_theme_id * omit default theme from child themes dropdown and try a different fix * cache & freeze stylesheets arrays