Rename template editor addon

The template editor is implemented by the 'hippo-ecm-addon-template' addon.
The reason why it was called this way is because of the naming convention that the plugin classes for addon foo are all in the package org.hippoecm.frontend.plugins.foo
The template editor is in org.hippoecm.frontend.plugins.template

However the name 'hippo-ecm-addon-template' suggests that it is a "template for addon creation" which it obviously isn't

Renaming the addon creates a huge diff and should be done at an appropriate moment (when nobody has any outstanding work)