Style the Admin Panel with a modern theme and Khoj branding (#999)

Overview
- The default django admin panel UI looks pretty dated and didn't
  have any Khoj specific branding
- Used the Unfold Django admin panel theme for a modern look
- Used the Khoj logo and name in Admin panel title, headings, favicons

Details:
All models shown on Admin panel need to inherit from unfold's
ModelAdmin to get styling applied. So

- Make all models on Admin panel inherit from unfold's ModelAdmin
- Subclassed UserAdmin to inherit from unfold's ModelAdmin
- Deregistered the unused Auth Group model from the Admin panel
  We can add it back when its actually used. Avoid confusion for now
- Explicitly register DjangoJobExecution on admin panel and again make
  it inherit from the unfold.admin.ModelAdmin

pyproject.toml

@@ -64,6 +64,7 @@ dependencies = [
     "anyio == 3.7.1",
     "pymupdf == 1.24.11",
     "django == 5.0.9",
+    "django-unfold == 0.42.0",
     "authlib == 1.2.1",
     "llama-cpp-python == 0.2.88",
     "itsdangerous == 2.1.2",