[49574871] more functionality support for 'assign candidates'

1 Don't list candidates that are already assigned to the selected opening;
2 limit the description's length to 200 for better readiblity, which shows within the expanded area.

Signed-off-by: Henry Chen <hchen@vmware.com>
Merge pull request #177 from Henry-CF/assign_candidates

[49574871] tune up of dialog 'assign candidates'