Add package-files-are-editable dev flag

This is to facilitate debugging of package-related issues.

One thing that doesn't work currently: if a package file is open while the flag is off, and then the flag is turned on and CDE is restarted, the reloaded source is still read-only. It needs to be closed and reopened to make it writable.
Merge remote-tracking branch 'upstream/master' into add-flag-to-edit-package-sources

Conflicts:
	ide/app/lib/spark_flags.dart
Add a default for new flag to app.json
Merge pull request #3320 from ussuri/add-flag-to-edit-package-sources

Add package-files-are-editable dev flag