Improve checkout command UX, and some extras

platform checkout:
* remove the `--project` option because it doesn't make sense to run this outside the project root
* remove `--environment` option because the command doesn't use it
* don't bother to fetch from origin if the branch already exists locally (in that case this works offline), otherwise, explicitly check whether the environment really exists before trying to fetch
* offer a choice of environments if none is specified
* use 'check out' as the verb in descriptions (http://notaverb.com/checkout)

platform branch:
 * list environments if there are no arguments (like 'git branch')

platform get:
 * use question helper instead of deprecated dialog helper when offering a choice of envs

platform backup:
 * 'backup' is not a verb (http://notaverb.com/backup)

other commands:
 * change 'id' (referring to a part of consciousness) to 'ID' (referring to an identifier)