Remove "Backpack settings" site administration page and improve UI

Site administration page for "Backpack settings" can be removed in order to improve UX:

• External backpack connection can be moved to "Badges settings". If it is disabled, "Manage backpacks" shouldn't be displayed (as happens now).
• "Active external backpack" can be removed. The default external backpack will be the first in "List of backpacks" ("Site administration > Badges > Manage backpacks"). So an option for moving up/down should be added (in order to let admins set the default).