Rework settings checks

Created by: chrismwendt

These two PRs:

• https://github.com/sourcegraph/sourcegraph/pull/1863
• https://github.com/sourcegraph/sourcegraph/pull/1879

caused a bug in migration that was reported in these issues:

• https://github.com/sourcegraph/sourcegraph/issues/1887
• https://github.com/sourcegraph/sourcegraph/issues/1888

so I reverted them in https://github.com/sourcegraph/sourcegraph/commit/9590c9ee310d5b6d8645a1159d1dc026453af93c and I'm bringing them back in this PR with slight modifications to avoid breaking migrations.