_This change should have no user impact._
The database corruption flag lives on `UserDefaults`. Currently, this
flag is controlled through `SSKPreferences`, but I kinda think that's
the wrong place—database corruption isn't really a user preference.
This moves it into its own file, tests it, and [drops booleans in favor
of an enum][0]. I think this is useful on its own, but also prepares us
for an upcoming change.
[0]: https://www.luu.io/posts/dont-use-booleans
