Change repository "Clone/Checkout As" to "Short Name"

Summary:
Ref T4245.

• Rename "Clone/Checkout As" to "Short Name" in the UI.
• Allow any repository to have a short name, not just hosted repositories.

Test Plan:

• Ran migration.
• Reviewed old transactions, saw they looked good.
• Edited an existing repository's short name.
• Gave an imported repository a new short name.
• Removed a repository's short name.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4245

Differential Revision: https://secure.phabricator.com/D14989