Allow users to customize their page

[erikaheidi]

Allow users to change the background color or upload a background image to their links page. Then, it would be interesting to also allow for changing the "theme" / style to light or dark mode.

This will require adding a new form section in the profile settings page, and update the user's table migration / model to have the additional fields.

We can probably use the profile upload image form as base for the background upload.