Try updating settings once more if there was a version mismatch

Created by: chrismwendt

Previously, if there was a version mismatch when updating settings, an error would be thrown and the settings would not update. No further settings updates would work until a page refresh.

This adds 1 retry and fetches the new settings just before attempting to update the settings once more.

Fixes https://github.com/sourcegraph/sourcegraph/issues/1474 Fixes https://github.com/sourcegraph/sourcegraph/issues/1278