Remove description from onboarding, allow users to change it in settings

Changes for https://github.com/uProxy/uproxy/issues/637

* Remove description screen from onboarding
* Keep description in instance messages, but default it to empty
* Add the ability to change the description while a user is signed in, on the settings screen
* In the buddylist:
 * If a buddy only has one instance, show the instance name if it's set, otherwise don't show any instance description.
 * If a buddy has multiple instances, show the instance names where they are set, otherwise default to names like "1st Computer", "2nd Computer", etc.

Tested in Chrome, Firefox (UI loads) updated grunt test
Can we call them ```Computer 1```, ```Computer 2```, etc. to do less work & not set ourselves up for translation failures?