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

[JasonGrace2282]

dark/edit.css should only be loaded wherever edit.css is loaded. The way I suggest doing this is adding a context processor that adds the context variable dark_mode_enabled to every template, and use that to optionally load themes.
More information here
https://docs.djangoproject.com/en/4.2/ref/templates/api/#using-requestcontext.
Then, wherever edit.css is loaded, just do

{% if dark_mode_enabled %}
  {# link dark/edit.css #}
{% endif %}