Allow all users to use changes /index REST API

The /index changes REST API endpoint requires the user to be a member
of a group having the Administrate Server permission.

This is too restrictive.  Users should be able to force indexing of
a change without being an administrator.

Allow /index to be used by any user that can see the change, i.e. has
read permission on the destination branch.

Change-Id: I3734e123a2859fcd0197895c42f9676a411d5716
3 files changed