Team model

[dchau2017]

Here is the specification for an Team model. For context on what the functionality will be here, we will have an entry for each team for each semester.

This way, we can keep track of what the current teams are and who is working on which project each semester.
Team

• title (string)
• user_ids ([ref])
• semester_id (ref)
• project_id (ref)

The role of this issue is to create the needed database model so that Mongo will know what properties a 'Team' document should have (basically, just the 4 properties we have above).

The model for posts may be a good reference (found in server/api/post/post.model.js).