App: Stats

Requires server changes here: selene-server!1 (merged)

Adds the following features:

• New "prerelease" user optional setting, which is for power users between Experimental & regular release
• Explore page gated behind Prerelease for the time being, additional Explore options behind Experimental
• Selene stats block & active user charts updated
• Also added some missing translations to the Debug and Error screens

Note: Check ins happen on any prerelease enabled device. Before upgrading to main release, I will also add a setting to switch off daily check ins and explaining about how it works for users concerned with privacy.