Enforce sensible, unique clone/checkout names for repositories

Summary:
Fixes T7938.

• Primarily, users can currently shoot themselves in the foot by putting ../../etc/passwd and other similar nonsense in these fields (this is not dangerous, but also does not work). Require sensible names.
• Enforce uniqueness so these names can be used in URIs and as identifiers in the future.
• (This doesn't start actually using them for anything fancy yet.)

Test Plan:

• Gave several repositories clone names: a valid name, two duplicate names, an invalid, name, some with no names.
• Ran migrations.
• Got clean conversion for valid names, appropriate errors for invalid/duplicate names.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T7938

Differential Revision: https://secure.phabricator.com/D14986