[47869323] db migration to support JD's seat counting and candidates' multi assessment

1 For JD, we need support vacancy counting and filled counting.
2 We need support assessments multi-times for candidates after 1+ round of interviews.
3 Also cleanup some code.

Signed-off-by: Henry Chen <hchen@vmware.com>
rename migration file to indicate the purpose
Signed-off-by: Henry Chen <hchen@vmware.com>
Merge pull request #75 from Henry-CF/candidate

[47869323] db migration for JD's seat counting and assement