Remove mention of "external services" and use "sync" consistently

Created by: mrnugget

These were two things that caught my eye after the introducing of the new "Manage repositories" entry in the sidebar with the changed naming, where "external services" doesn't show up.

Changes:

• Use "Code host"/"Code host connection" instead of "External service"
• Use "sync" consistently (we had a stray "mirror") somewhere
• Use "Manage repositories" (as it's called in the admin side bar) instead of "Configure synced repositories"

Before:

After: