Wording of boot menu entries ("Live") is unclear

Originally created by @intrigeri on #11975 (Redmine)

As suggested by Chris Lamb <lamby@debian.org> on tails-dev in September, 2015, “Live” is hard to undertand. Then an interesting discussion about it happened on tails-dev@ and tails-ux, but in the end no consensus was achieved. It seems to me that most of the proposals on these threads are better than the current state of things, so IMO until -ux tells us what we should put in there, we can pick basically any of them, and improve things. This ticket is about doing that.

Feature Branch: feature/11975-boot-menu-wording

Parent Task: #10178 (closed)