feature(users): support non-binary genders

[smayya337]

Feature description

This is not a major priority, but currently, the User.gender field is set using a boolean - True for male, False for female. This becomes a problem when non-binary individuals are added to the mix, since that by definition is neither True nor False.

Rationale for adding feature

TJ has a non-negligible non-binary population.

Suggested implementation(s) (if applicable)

Could we change User.gender to be a string instead of a boolean?

[etnguyen03]

Isn't Ion's gender field a nullable boolean field?

[smayya337]

I believe so, but we use the null option as a catch-all for everything not male or female (like, for instance, when we don't bother to have a gender at all) - I figured it might be useful to have another non-null option besides male and female, as opposed to just not giving non-binary individuals a bool gender.