feature flags: user settings feature flags overrides

Created by: bobheadxi

Part of https://github.com/sourcegraph/sourcegraph/issues/31182. Manage feature flags override from user settings

https://docs.sourcegraph.com/dev/how-to/use_feature_flags#feature-flag-overrides

Also see: https://github.com/sourcegraph/sourcegraph/issues/32233 , https://github.com/sourcegraph/sourcegraph/issues/32238