Provide more UI guidance when creating repositories

Summary: Ref T10923. Walk users through the "create, configure, activate" workflow a little better and set expectations more clearly.

Test Plan:

• Created a new repository, saw new UI help.
• Activated repository, saw onboarding help disappear.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10923

Differential Revision: https://secure.phabricator.com/D15875