Update CP > Users page to allow for 3 available site member roles #2761

#2761

### Summary

Replaces the old "Make admin" and "Remove admin" overlay forms with a single "Edit membership" form, with 3 role options.

### Changes

* The Manage Members page table now has 4 columns: "Name, Username, Role, Status"
![screen shot 2013-06-11 at 12 34 42 pm](https://f.cloud.github.com/assets/228733/638131/dbee0826-d2b4-11e2-83e7-ebe07f11279f.png)

* The "edit" link on the "Role" column brings up the new modal form in an overlay.
![screen shot 2013-06-11 at 12 36 39 pm](https://f.cloud.github.com/assets/228733/638142/1edfadba-d2b5-11e2-96bd-c4dc3d269ad3.png)

* Use build array to serve page content, not HTML string.
* Refactor helper functions to reduce complexity.
* Add button style & margin, and operation link float right styles to page via #attached