Finish creating disable_free_space_check variable

* Create data manager variable for 'tw_disable_free_space'
* Make configurable through Settings
* Fix the "two presses needed to enable" issue for the checkbox on the
  backup screen
* No need to capitalize every word, but do specify precedes backup

Change-Id: Id436cef13e4ca9349618420aac03862ec4e3c35e
diff --git a/data.cpp b/data.cpp
index 1ea7b84..81e9ca2 100644
--- a/data.cpp
+++ b/data.cpp
@@ -679,6 +679,7 @@
 	mPersist.SetValue(TW_REBOOT_AFTER_FLASH_VAR, "0");
 	mPersist.SetValue(TW_SIGNED_ZIP_VERIFY_VAR, "0");
 	mPersist.SetValue(TW_FORCE_MD5_CHECK_VAR, "0");
+	mPersist.SetValue(TW_DISABLE_FREE_SPACE_VAR, "0");
 	mPersist.SetValue(TW_USE_COMPRESSION_VAR, "0");
 	mPersist.SetValue(TW_TIME_ZONE_VAR, "CST6CDT,M3.2.0,M11.1.0");
 	mPersist.SetValue(TW_GUI_SORT_ORDER, "1");