API console: indicate presence of more completions

Created by: sqs

In the API console, when using completion, it is not clear that the list scrolls and that there are more options:

Repro: Go to https://sourcegraph.com/api/console and trigger completion (eg by pressing Ctrl/Cmd+Space).

We use https://github.com/graphql/graphiql for the API console, and it uses the CodeMirror editor. I see that it already applies the overflow-y: auto in CSS to the menu, which should make it at least flash a scroll bar, but it's not doing so.

• Requested by customer https://app.hubspot.com/contacts/2762526/company/407948923