Use /users/:username/repos endpoint for GitHub mirroring

When determining the mirrorable repositories for a user, ScriptRunner currently uses the /user/repos endpoint of the GitHub API, which implicitly chooses the user based on the provided access token. However, the endpoint only shows repos which that user has explicit access to: this can miss out some public repositories, and also precludes the possibility of using one user's token to mirror a public repo belonging to another user.

Organization-based mirroring uses the /orgs/:org/repos endpoint, which takes the name of the organization of interest as an explicit URL parameter. There is an analogous endpoint for users, /users/:username/repos, which ScriptRunner should use in place of /user/repos.