Remove unused anonymous_by_default column

[jasonaowen]

The User model has an anonymous_by_default attribute, which is used to fill in missing data in saving a nicety:

https://github.com/mjec/rc-niceties/blob/934a8919fac297399b160017898376feb14be997/backend/api.py#L319

It defaults to False: https://github.com/mjec/rc-niceties/blob/934a8919fac297399b160017898376feb14be997/backend/models.py#L25

However, as far as I can tell, there's no way for a user to change this preference. It doesn't seem to be exposed in the API, and there are no references to it in the front end, so I think this part of a partially implemented feature.

In the existing data, there are about 10 signed niceties for every anonymous nicety, which suggests that making anonymous niceties is a relatively uncommon activity. This is a small design decision, but I would prefer to make it easier to write niceties that the recipient can see faces next to, rather than pave the way to giving anonymous feedback in bulk. This is also a technical decision: fewer features means less code to write and maintain.

Remove the anonymous_by_default attribute from the User model, and the corresponding column from the user table.