Make site extensions more CSM friendly

There is a bit of context that you'll need to know if you were not in the discussions, but here are the changes:

- rename extensions to siteextensions to avoid conflict with CSM 'Extensions' token
- Remove the /local/ segment so operations are top level
- Change extensions/remote to extensionfeed so it's top level
- Use PUT instead of POST for Install
- Pass the Id on the path instead of body for Install
- Support installing non-default versions