Resolve "Add verification to not put person in two groups at once"

When you join a group, add members to a group, and create a group with set members, there is a check to see if those members are already a part of a group. An error message is shown to the user if that is the case. Closes #113 (closed)

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assigned to @aturgut

added 19 commits

• 004ee142...a2cba8bb - 18 commits from branch development
• 1368fe01 - joinGroup and addMember checks if at least one student is already in a group

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added 1 commit

• 2754a129 - createGroup also checks if people are already in other groups

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marked this merge request as ready

changed the description

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• 9e398e14 - better dealing with empty username lists

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• 2d3627c8 - better error messages

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resolved all threads

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requested review from @dsavvidi

approved this merge request

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