Add exFAT support (#150)

## Summary

* Swap to updated SDCardManager which uses SdFat
* Add exFAT support
  * Swap to using FsFile everywhere
* Use newly exposed `SdMan` macro to get to static instance of
SDCardManager
* Move a bunch of FsHelpers up to SDCardManager

src/activities/network/WifiSelectionActivity.cpp

@@ -5,6 +5,7 @@
 
 #include <map>
 
+#include "MappedInputManager.h"
 #include "WifiCredentialStore.h"
 #include "activities/util/KeyboardEntryActivity.h"
 #include "config.h"