Fix an error stemming from too many things named belongsTo and hasMany. The ones on DS.Model currently expect the name of a model (like DS.belongsTo does), but it should expect the name of a relationship (like DS.Snapshot.belongsTo does).

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• Provide a URL to documentation or source code which provides context for the suggested changes: https://www.emberjs.com/api/ember-data/release/classes/DS.Model/methods?anchor=belongsTo
  https://www.emberjs.com/api/ember-data/3.0/classes/DS.Model/methods/hasMany?anchor=hasMany
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