external references UI support, panel improvements

Created by: sqs

• Adds UI support for showing external references from extensions. External references are now shown in the same panel as local references. If a reference is emitted that is from a different repo, another column will show up to let the user select the repo. The current repo is always at the top and initially selected.
• Adds a "Group by file"/"Ungroup by file" panel action that adds in another column to filter references by file. Extensions can add other things here, such as toggling on/off certain reference providers.
• Adds a new sourcegraph.languages.registerLocationProvider call that generalizes location providers and panels. This can be used to show locations that are neither definitions nor references (such as implementations, type definitions, etc.).

This is part of #1187 and a requirement for deploying #584 to Sourcegraph.com (the switchover to the new extension-based code intel).