Extract dev flags from a set of files + Switch to static API for SparkFlags to be able to override and append from new files
Merge 'upstream/master' + redesign SparkFlags once more

Conflicts:
	ide/app/spark_flags.dart
Add developer flags initialization from multiple files
Support per-user file for developer flags (survives `grind deploy`)
Read highest-precedence flags from <project_location>/.spark.json + refresh UI accordingly
Typo
Address review comments
Address review comments #2
Merge pull request #1815 from ussuri/improve-developer-flags

Improve developer flags