Dark mode Implementation

[hsna674]

Closes #2

• Adds a new field to the user model called dark_mode
• Adds the change_theme view to the user views.

[JasonGrace2282]

As discussed in person, can you add a test for the view to make sure it changes the dark mode?
Information on writing tests can be found on the website: https://tjcsl.github.io/tin/contributing/tests.html