Replace custom reading Implementation-* from MANIFEST.MF by java.lang.Package methods

There are some that have been reading Implementation-* attributes from the MANIFEST.MF resource manually by manipulating relative URL path using ../../../../, which doesn't seem to be very safe, as explained in HSTTWO-3890.

I'd like to propose changes in the following:

• org.hippoecm.repository.impl.RepositoryDecorator#getDescriptor(String) reads Implementation-Version manually.
  This should be replaced by getClass().getPackage().getImplementationVersion() simply.
• org.hippoecm.repository.StatusServlet uses RepoUtils.getManifestURL(getClass()) to read MANIFEST.MF input stream and reads Implementation-Version and Implementation-Build manually, as a fallback if there's no custom overriding MANIFEST.MF in the classpath.
  This uses RepoUtils#getManifestURL().
  As a conservative approach, we should use StatusServlet.class.getPackage().getImplementationVersion() if the implementation version is not found from MANIFEST.MF in any cases.