Mark ApprovalCopier, ApprovalsUtil and LabelNormalizer as singleton

These have no per-request state and are held by several REST API views
that are themselves singletons.  Instead of making one copy per REST
view, make these singletons for the server.

Change-Id: I2f343d29c4bed8a862b3794323916ca46b8b1f99
3 files changed