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.)

• 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.