Make all lists have uniform design

Created by: francisschmaltz

Some lists in Sourcegraph have missing top or bottom borders. All lists should have both a top and bottom borer and be uniform in list border design.

3.0 list updates

• Site-admin Auth providers (missing top border on :first-of-type)
• Site-admin External accounts (missing bottom border on :last-of-type)
• Site-admin Access tokens (missing top border on :first-of-type and missing bottom borer on :last-of-type)
• Site-admin External services (missing bottom border on :last-of-type)