Improve config problem reporting and site alerts

Created by: unknwon

This PR includes changes on management console that validates critical config on user update, and more clear error message with site alerts. Currently it only validates "externalURL" specifically for https://github.com/sourcegraph/sourcegraph/issues/3899. Also includes changes from https://github.com/sourcegraph/sourcegraph/pull/5555.

Unit tests are not added because I want to first make sure I'm doing it correctly.