pkg/db/globalstatedb: add management console password state / generation
Created by: slimsag
Depends on #1287
This PR adds to our global DB state the management console password. How this password will be used is outlined in https://github.com/sourcegraph/sourcegraph/issues/965#issuecomment-441552016
Actual usage of this password for authentication of the management console will come in a later PR. In the mean time, this poses zero security risk and as such I will merge quickly. @beyang @nicksnyder please review post-merge.