feat(repo): split repository creation limit into user and org scopes (#37872)

## Background

`MAX_CREATION_LIMIT` applies to whoever owns a new repository, with no
distinction between individual users and organizations. Admins who want
different limits for the two - most commonly "block personal repos but
let orgs create freely" - currently have to set per-user / per-org
overrides on every entity.

## Changes

Adds two new `[repository]` settings:

- `USER_MAX_CREATION_LIMIT`: global limit for individual users
- `ORG_MAX_CREATION_LIMIT`: global limit for organizations

`MAX_CREATION_LIMIT` is kept as a shortcut: when set, it becomes the
default value for both new keys. When the new keys are explicitly
configured, they take precedence. Deployments that only set
`MAX_CREATION_LIMIT` see behavior identical to now.

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>