clean up check selection code

This new approach allows filtering out checks from the default set that are
not appropriate for a command, rather than having to list every check
that is appropriate. It also reduces some boilerplate.

Haskell does not define Eq for functions, so I had to go a long way around
with each check having a unique id. Meh.

Command/Migrate.hs

@@ -16,9 +16,8 @@ import qualified Command.Add
 import Backend
 import Logs.Web
 
-command :: [Command]
-command = [Command "migrate" paramPaths defaultChecks seek
-	"switch data to different backend"]
+def :: [Command]
+def = [command "migrate" paramPaths seek "switch data to different backend"]
 
 seek :: [CommandSeek]
 seek = [withBackendFilesInGit start]