feature request: read-only or service-specific access tokens

Created by: slimsag

A user has asked that we support read-only or service-specific access tokens. For example, I wish to put a Sourcegraph access token in our CI system but do not want it to have access to modify my user settings or anything else.

Related: https://sourcegraph.slack.com/archives/C0W2E592M/p1573502571034500