Improve the usability of the external services UI

Created by: beyang

Improves the usability of the external services UI with the following changes:

• Declutter the "add external services" page, and remove "external services" in favor of "add repositories".
• Add explicit instructions for each external service and declutter the default config. We previously relied on default config to provide de facto documentation, and this meant a lot of clutter and verbiage in the config box by default.
• Add a "repositories" subsection to the site admin sidebar and remove "external services" in favor of "add/view repositories" language.

The screenshots below provide examples of the above changes.

Before	After

This change also makes minor improvements to onboarding and the activation widget, but I'm holding off, because I would like to make larger scale changes to the onboarding experience.