Update Greeter icons

The icons could be updated to the most recent ones.

https://labs.riseup.net/code/issues/8523
https://mailman.boum.org/pipermail/tails-ux/2015-December/000835.html

Parent Task: #10828 (closed)

Related issues

• Blocked by #8523 (closed)

Original created by @SpencerOne on 11472 (Redmine)