Updates to Enhanced User groups design proposal

- separated out "Modify Jobs" as a distinct per-member permission
- made it clear that this builds on the existing groups support
- made it clear that LDAP groups do NOT allow per-member settings
- added UI sketches for updating group details
- group-manage command renamed to group-modify
- group description field added
- data migration guide added for existing Beaker installations

Change-Id: I29e93329d94b56a196b00e511391b85d41ec90ee