[Minor Refactor] Unify RepoType similar methods

What does this MR do and why?

This is just a minor refactor MR.

Use define_method to unify some similar methods in RepoType.

Extracted array SUPPORTED_NAMES is more readable.

Use comments to make code search easier:

    # Defines instance methods:
    #   - project?
    #   - wiki?
    #   - snippet?
    #   - design?

cc JiHu EM @prajnamas