frontend: Add custom field serializer for RepoImage

This is a workaround to resolve some current issues with GWT RPC
serialization.

Change-Id: I137d47dee58dce90a66a3c4accfe75e146223749
Bug-Url: https://bugzilla.redhat.com/988021
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>