Allow installation to more than one drive

This commit somewhat overhauls the partition selection screen.
It introduces the following changes:

* parted binary is used for easy, optimal partitioning;
* the HDD selection screen is no more,
* instead, all attached drives (sans the rootfs one) are shown
  in the disks (partitions) treeview,
* and there is a button 'Expert mode' for the "custom mount onto
  /target instructions";
* all python code related to partitioning is in the new
  partitioning module;
* the colorful partitions webview is a standalone HTML file in
  usr/share;
* plus other related minor simplifications, additions & fixes.

Also closes #21.