Allow preview sites to be enabled and disabled; only show enabled sites as preview options

Hello! I noticed that the config for each Preview Site includes a status value, but as far as I could tell, nothing allowed me to change that value in the admin UI, and if I manually edited and imported the config to disable the site, it didn't do anything.

I'm adding a merge request that adds a checkbox to the Preview Site edit form to enable or disable a site. It also adjusts the Decoupled Preview form on the front end so that the site selection dropdown only includes enabled sites.