Fixes for Ushahidi 2.8

* Extend Main_Controller instead of Template_Controller
* Stop reinitializing Themes library.
* Adminmap: Enabled map and slider in Themes library
* Adminmap: Pass themes library reference to set_map()

Also still requires this fix to core: https://github.com/ushahidi/Ushahidi_Web/commit/253b57aa2d0fc834a2ff684c257e3cb9bca9668d