Fixes CLA Signing and Trivial Patch Exemption links

The changes in this PR fix the CLA Signing and Trivial Patch Exemption links in the `CONTRIBUTING.md` file. These changes are necessary because we recently moved the application used to manage the CLA signing process (which is also the application that contains the documentation on things like what makes up a Trivial Patch) from the `c.mobileins.us domain` to the new `opensource.csc.com` domain. This PR should be merged as soon as possible, because without the changes in this PR users will be directed to a non-existent page for the CLA signing process...