Language support via Sourcegraph extensions

Created by: sqs

See https://sourcegraph.slack.com/archives/CCLF4R6EM/p1540856089027900 for context.

This PR updates the CHANGELOG.md file to describe any user-facing changes. (TODO)

TODOs

• Ensure that Go symbol URLs (/go/-/...) and godoc.org links still work
• File a followup issue to remove the repo.indexed_revision and repo.freeze_indexed_revision columns later https://github.com/sourcegraph/sourcegraph/issues/644
• Remove queryLSP from @sourcegraph/extensions-client-common
• Selectively show implementations panel if an implementations provider is registered
• Show external references separately?
• Assess consequences of removing noGoGetDomains and the go get domains blacklist - add back to Go extension https://github.com/sourcegraph/sourcegraph/issues/1276
• Find an alternative data source for badges (ref count)
• Remove LSP compat code in isEmptyHover, renderHoverContents