Add `sex` field to Users and implement `honorifics` method

[gabrielshufelt]

Description:
We want to enhance the User model by adding a sex field and implementing a method to return honorifics based on the user's selected sex.

Requirements:

1. Add a new sex field to the User model, with the following options:
   • Male
   • Female
   • Other
2. Update the registration form to include a required dropdown for users to select their sex.
3. Ensure that a user must choose one of these options to successfully register.
4. Implement an honorifics method in the User model, which will return an appropriate honorific string based on the user's sex:
   • Male: "Mr."
   • Female: "Ms."
   • Other: "Mx."
5. Ensure that calling User.last.honorifics in the rails console will return the appropriate honorific for the most recently created user.

Acceptance Criteria:

• [x] #37
• [ ] Registration form includes a required dropdown for sex with the options: Male, Female, Other.
• [ ] The honorifics method returns "Mr.", "Ms.", or "Mx." based on the user's sex.
• [ ] Tests are added to ensure validation and behavior of the sex field and honorifics method.
• [ ] Documentation is updated to include the new sex field and honorifics method.