web: Add external service cards to repo settings page

Created by: keegancsmith

We include links to the external service settings for a repository. This is only shown on the repository settings page, which is only visible to admins. The list of external services returned from the backend is non-empty iff the repository is managed by our new syncer. For repositories not managed by our new syncer, we still show the disable and delete repository actions.

Test plan: Locally had GitHub repositories with a single external service and two external services. Visited the repository settings page for those repositories and clicked through to the external services. Additionally tested against a gitolite repository to ensure it still showed the disable/delete actions.

Part of #2025 (closed)

Mocks

• Sourcegraph Engineer / Design Spec Link: https://zpl.io/anpXvGr
• Public / Non-Engineer Design Link: https://sketch.cloud/s/wwoGV/Axae1bR/play serving requests.

State

• This PR does not implement the collapsing of the settings page. That is for another PR.
• Need to add the > to the end of the card to indicate it is clickable. See mocks.