feature purge: Implement a new subcommand

The new subcommands deletes all feature branches and base markers that are
associated with a story that is in the Accepted state. The user is asked for
every local and remote branch to confirm that the branch is supposed to be
deleted.
Finished feature 76779734/command-purge-branches.
Merge branch 'master' into develop
review: Fix Unicode issues once and for all
Finished feature 77876896/fix-unicode-in-story-name.
Update rbtools to 0.6.2
Finished feature 78009130/update-rbtools.
Bump version to 1.4.8
review: Fix some lasting unicode issues
Finished release 1.4.8 (merge into master)