Fix calculation of scoped setting defaults

[winstliu]

Requirements

• Filling out the template is required. Any pull request that does not include enough information to be reviewed in a timely manner may be closed at the maintainers' discretion.
• All new code requires tests to ensure against regressions

Description of the Change

Prior to this PR, the scoped setting default was ignored and the unscoped default was used instead. That was done in #841 to fix #774 (not being able to change scoped settings if the unscoped value was also changed). However, that caused a different bug where if the scoped default was different than the unscoped default, it was impossible to change the scoped value to the unscoped default. This PR aims to fix both issues. First, we search for the scoped default (without looking at the config.cson). Then we check the unscoped default. If the unscoped and scoped defaults are the same, we then further look at the config.cson to determine the currently-set unscoped value, which effectively acts as the scoped default.

Alternate Designs

None.

Benefits

Editing scoped settings should work as expected now.

Possible Drawbacks

Hopefully none.

Applicable Issues

Fixes #939
Refs #774