refactor: App.hs and related changes (#1725)

* Use ExceptT to avoid 'staircasing' case analysis in App.hs
* Split large function in App.hs into individual handler functions
* Adapt API of Auth.hs, OpenApi.hs etc. to simplify the use of those modules in App.hs
* Split optional rollback functionality into Middleware
* Unify SimpleError and ApiRequestError into one Error type, so it can be used across modules

test/SpecHelper.hs

@@ -22,8 +22,7 @@ import Test.Hspec
 import Test.Hspec.Wai
 import Text.Heredoc
 
-import PostgREST.Auth   (parseSecret)
-import PostgREST.Config (AppConfig (..))
+import PostgREST.Config (AppConfig (..), parseSecret)
 import PostgREST.Types  (JSPathExp (..), LogLevel (..))
 import Protolude        hiding (toS)
 import Protolude.Conv   (toS)