In preparation for unified functions, added `moveFile` and `moveFolder` functions:
+ This will make the API for move consistent with copy and delete.
+ It will allow us to create a unified move function independent of existing
  functionality.

The new `copy` function unifies `copyFile` and `copyFolder`.
It determines whether the source is a file or folder from the server response (without the need for an extra network request) instead of the trailing slash on provided source url.

Reuses prior GET and PUT responses to get links for copying.
This saves unnecessary extra HEAD calls being made to fetch links. Logic for copying links remains unchanged.

The error handling for inputs is placed in its own function, which tests:
+ if inputs are strings
+ if source and destination are not the same
+ if the source is not a parent of the destination

The new `remove` function unifies both `deleteFile` and `deleteFolder`.
(It can't be called `delete`, which is already reserved by the low level API)

It determines whether the source is a file or folder from the server response
(HEAD call is reused in case of file deletion) instead of the presence of a trailing slash on provided source url.

The new `move` function that builds upon the unified `copy` and `remove` functions
It avoids the unnecessary duplication of work that resulted from running copy and
delete separately.