Edit backup_and_restore.rst
Edit backup_and_restore.rst

- make various changes for style and clarity throughout
- change the title this topic guide to "Back Up, Restore, Migrate"

ehartsuyker/securedrop,conorsch/securedrop,conorsch/securedrop,heartsucker/securedrop,ehartsuyker/securedrop,conorsch/securedrop,heartsucker/securedrop,conorsch/securedrop,heartsucker/securedrop,ehartsuyker/securedrop,heartsucker/securedrop,heartsucker/securedrop,conorsch/securedrop,ehartsuyker/securedrop,ehartsuyker/securedrop,ehartsuyker/securedrop