Move ChangeKind to PatchSetAttribute

When adding the ChangeKind for JSON data, I added it into
PatchSetCreatedEvent which ended up being a pretty lousy place to
add it, since it has little to do with the event, and more to do
with the PatchSetAttribute itself.  This moves the data to the
PatchSet JSON structure, alongside the draft attribute.

Change-Id: Ia3791789af2ddb6efc70d78f38595519b5d6ab5f
Remove kind attribute from REST containers

It seems like new REST endpoints don't include "kind" attribute. This
change removes it from the code and documentation.

Change-Id: Ia73fb92ce01461e7de140b138fc72583b00170bc
Merge "Move ChangeKind to PatchSetAttribute"