normalize functionality of `onyo new --clone` and `onyo new --template`

[TobiasKadelka]

In my opinion, the major difference between onyo new --clone and onyo new --template should be how they tab-complete. Functionaly they are doing the same: let the user specify a yaml file that is used as a base for the new asset.

In practice, there are different limitations where we disallow certain locations for the yaml files, like in #775.

I propose that onyo new --clone and onyo new --template should both allow to use a yaml file from anywhere as a base for the new asset, and only are different in regards to tab-completion. And indeed, onyo new --clone does allow this basically for any file (it only misses to check if the file actually exists, see #774).

[aqw]

This is all being changed as part of Ben's work on --yaml. Huge reform and mostly unification of how --clone, --template, --yaml, and onyo show will behave.