Merge pull request #24 from KimMinJeong/board_main_contents

Board main contents
Merge pull request #27 from KimMinJeong/board_main_contents

Board main contents
Modify DB relation
modify DB relation
Entirely Modify DB relation
cascade 수정
Merge pull request #28 from KimMinJeong/KMJ

Entirely Modify DB relation
Merge branch 'master' of github.com:KimMinJeong/Collabo into board_main_contents

Conflicts:
	Board/Board_main.py