Add a fact checker feature with updated styling (#835)

- Add an experimental feature used for fact-checking falsifiable statements with customizable models. See attached screenshot for example. Once you input a statement that needs to be fact-checked, Khoj goes on a research spree to verify or refute it.
- Integrate frontend libraries for [Tailwind](https://tailwindcss.com/) and [ShadCN](https://ui.shadcn.com/) for easier UI development. Update corresponding styling for some existing UI components. 
- Add component for model selection 
- Add backend support for sharing arbitrary packets of data that will be consumed by specific front-end views in shareable scenarios

src/khoj/database/models/__init__.py

@@ -359,3 +359,10 @@ class EntryDates(BaseModel):
 class UserRequests(BaseModel):
     user = models.ForeignKey(KhojUser, on_delete=models.CASCADE)
     slug = models.CharField(max_length=200)
+
+
+class DataStore(BaseModel):
+    key = models.CharField(max_length=200, unique=True)
+    value = models.JSONField(default=dict)
+    private = models.BooleanField(default=False)
+    owner = models.ForeignKey(KhojUser, on_delete=models.CASCADE, default=None, null=True, blank=True)