Checklist for 3.0-preview on Sourcegraph.com

Created by: chrismwendt

Tasks that must be completed before removing lsp-proxy from Sourcegraph.com and switching over to extensions for everything:

• @chrismwendt lang-go: Increase concurrency of external references
• @sqs Sourcegraph extension API: add support for extensions emitting external refs https://github.com/sourcegraph/sourcegraph/pull/1191
• @chrismwendt lang-go: uncomment registerExternalReferencesProvider (in addition to the normal references provider)
• @chrismwendt /go/ links: reimplement using https://golang.org/pkg/go/doc/#Package.Filter https://github.com/sourcegraph/sourcegraph/pull/1197
• @chrismwendt Add HTTPS support to gddo-proxy https://github.com/sourcegraph/infrastructure/pull/752
• @chrismwendt Fix erroneous assumption that import paths are the same as repos in lang-go
• @ijsnow Codecov extension: disable by default; run migrations to enable for anyone who has entered a token
• @ijsnow Browser extension: always use extensions when Sourcegraph URL is Sourcegraph.com (disregarding the checkbox)
• @ijsnow DB migration to enable the Codecov extension for any user that also has other codecov.* settings
• @sqs Blog post https://github.com/sourcegraph/about/pull/64 and tweet
• @sqs Python #959 (closed)
• @sqs Enable basic code intel for all users https://github.com/sourcegraph/sourcegraph/pull/1240 https://github.com/sourcegraph/sourcegraph-basic-code-intel/pull/8 https://twitter.com/srcgraph/status/1070628324542603265
• @sqs add more basic-code-intel langs