Implement the ability to add/remove users to/from a group

[RTEYL]

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Similar to #69

Add Redux actions for groups to add/remove a user. The way it is now, the group being updated is reliant on the front end to send the updated group to Redux instead of simply calling a function with the group and user data to be handled on the backend.

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Create an action-type, action, and reducer case function that accepts arguments for a group and a user. Then send a request to update Mongo and the store.

Needs one for adding a user and one for removing a user.

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