Admin RFP toggle

Issue: #724

> As a member of Bountysource
> When a team has requested RFP's be enabled for them
> Then we go to the team page
> And I press "Enable RFP's for this team"

* Render a panel if current user is a Bountysource admin (`people.admin`) with "Enable RFP" button.

![image](https://cloud.githubusercontent.com/assets/692632/3098973/773c8ca0-e5f4-11e3-8302-d0d50876ad71.png)


* Teams update action accepts `rfp_enabled` as a boolean param, requires current user to be admin to complete update if that attribute is modified.