User profiles: let users have public bios

We would like to have public bios for users/hosts. We'll need to add a public bio field to the `users` model and surface that field throughout the application.

- [ ] Add a field for public bio to the `users` model
- [ ] Modify the `search-hosts` API endpoint to return the user's public bio
- [ ] if #78 is done, add a field to edit the public bio on the Add User and Edit User forms
- [ ] Add the public bio to the user profile page