Data loading: make community name the object id

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Small change, see notes.

• [x] Add _id to the Community struct in models.rs
• [x] Change this line in main.rs

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Tested this and the _id has changed, but there are still duplicate objects in the database.

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See about creating a compound index https://www.mongodb.com/docs/manual/core/timeseries/timeseries-secondary-index/