Give users a modal VCS choice when creating a new repository

Summary:
Ref T10748. Allow the new EditEngine workflow to create repositories by giving the user a modal repository type choice upfront.

(The rest of this flow is still confusing/weird, though.)

Test Plan:

• Created a new repository.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10748

Differential Revision: https://secure.phabricator.com/D15813