@WIP: Change "manager" to "site owner" in people tab and add "change" link.

#5311 

WIP

Changed "manager" to "site owner" in ``<vsite>/cp/users`` and added a "change" link to change the owner of the site:
![selection_023](https://f.cloud.github.com/assets/4497748/2470695/22265dbc-b017-11e3-9cbe-4c004c48590e.png)

Pressing the "change" link opens a form in a modal:
![selection_024](https://f.cloud.github.com/assets/4497748/2470697/27db9c5e-b017-11e3-977a-1b383960d8a1.png)

To do:
 - Make the autocomplete field on site level (needs to autocomplete users from the same site).
 - Limit access to ``cp/users/change_site_owner`` only to the site owner.