Review "unique" fields in database

[dsj976]

Some fields of the database table that are not currently defined as unique should be reviewed. For instance, the email field of the user table is not defined as unique because it was originally thought that this would allow the same annotator to create different accounts with different usernames, but the same email address, for different annotation campaigns. Upon reviewing this, it does not make much sense because a more consistent way of annotating the same dataset twice would be to upload the same dataset but with a different name.

TODO:

• [ ] Review the unique fields in the database
• [x] Allow for two different datasets to have the same name, as long as they are not assigned to the same annotator. In other words, the names of the datasets assigned to a particular annotator should be unique - fixed with bc0218c

[dsj976]

closing this as is a duplicate of #33