MongoDB backend connection changes

added 12 commits

• 628ab992...ef726734 - 9 commits from branch master
• 1b5eed62 - Merge branch 'master' of gitlab.engr.illinois.edu:nanyiry2/team-tbd-cs411 into yutingd3
• 9897b0f1 - Merge branch 'master' of gitlab.engr.illinois.edu:nanyiry2/team-tbd-cs411 into yutingd3
• c48ae8dc - connect mongDB with backend

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added 1 commit

• 274aab53 - connect MongoDB with backend

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closed