Add New Case | Demographic Details | Gender selected as Other | Text input box should appear.

### Background on the problem the feature will solve/improved user experience
When entering a new case, demographic details restricts the Gender dropdown options to Male, Female, Other, Unknown.

### Describe the solution you'd like
When adding a new case and enter demographic details. When I select Other for Gender, I should be presented with a text box that gives me the option to enter the Gender.
The new text box should be displayed either below or beside the Gender dropdown. The user can optionally enter text into the text box if they want to specify otherwise the value will return "Other".

### Tasks
Description of the development tasks needed to complete this issue, including tests,

### Acceptance Criteria
Standards we believe this issue must reach to be considered complete and ready for a pull request. E.g precisely all the user should be able to do with this update, performance requirements, security requirements, etc as appropriate.

Locate the new-case demographic details form and the Gender dropdown, then inspect how the selected value is stored and submitted. Add the requested conditional text input for “Other” and verify that an optional custom value is submitted, otherwise the value remains “Other”.

Written by the indexing model from the issue text.