Enable UI handling logic with dev passthrough directories. (#145)

* Enable UI handling logic with dev passthrough directories.

* Add tests and fix a couple of things

internal/cmd/commands/controller/dev_flags.go

@@ -13,6 +13,6 @@ func addDevOnlyControllerFlags(c *Command, f *base.FlagSet) {
 		Name:   "dev-passthrough-directory",
 		Target: &c.flagDevPassthroughDirectory,
 		EnvVar: "WATCHTOWER_DEV_PASSTHROUGH_DIRECTORY",
-		Usage:  "Enables a passthrough directory in the webserver at /passthrough",
+		Usage:  "Enables a passthrough directory in the webserver at /",
 	})
 }