src login command to help users configure authentication w/access token

Created by: sqs

You can run src login https://sourcegraph.example.com to verify that your src is authenticated to access that Sourcegraph instance. If not, it'll show you friendly steps to help you get it set up.

This will simplify all of our docs and in-product text that relies on users using the CLI. The steps to get them to the right state will be just:

1. Install the Sourcegraph CLI (src) at https://github.com/sourcegraph/src-cli [or link to releases]
2. Run src login SOURCEGRAPH_URL [and we can even fill in SOURCEGRAPH_URL in in-product text]

Here is a screenshot of what it looks like in various states:

README section: