refactor: move isolation.active from preferences to storage

[rkodey]

Centralize isolation state management and storage access Add isolation helper functions getActiveState, toggleActiveState Retire unneeded helper function handleActiveState Initialize isolation before browseraction Related to #315 Closes #433

[stoically]

Code looks good so far, thanks! Have to think a bit about whether active should be in the preferences or not, I'll come back to it ASAP.