Rework the hosts assignment page

Update table design for Deployment Roles according to wireframes (page 26)
Prevent table of hosts to be always visible
Refine "assigning hosts" workflow (page 7-11)
Refine "unassigning hosts" workflow (page 12-16)
Allow user to see deployed hosts for each role (page 26, note #6)
Enable user to close hosts table
It should allow to unassign a host when `!deployment_in_progress` && `host.open_stack_deployed?` but with a warning.