fix two candidates bugs

[50584785] The quitted candidates shows in 'schedule interviews' page's combobox
[50159691] refine 'candidate detail/index' page to show overall status of the candidate
[50639633] status cannot be changed in 'final assessment' dialog

Signed-off-by: Henry Chen <hchen@vmware.com>
Merge branch 'master' of https://github.com/vmw-tmpst/heroapp-LetsHire into candidate

Signed-off-by: Henry Chen <hchen@vmware.com>

Conflicts:
	app/models/opening_candidate.rb
Merge pull request #213 from Henry-CF/candidate

fix several candidates bugs