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/util/KeyboardEntryActivity.cpp

@@ -1,6 +1,7 @@
 #include "KeyboardEntryActivity.h"
 
 #include "../../config.h"
+#include "MappedInputManager.h"
 
 // Keyboard layouts - lowercase
 const char* const KeyboardEntryActivity::keyboard[NUM_ROWS] = {