Add a unique_id field for identifiying conversations (#914)

* Add a unique_id field to the conversation object

- This helps us keep track of the unique identity of the conversation without expose the internal id
- Create three staged migrations in order to first add the field, then add unique values to pre-fill, and then set the unique constraint. Without this, it tries to initialize all the existing conversations with the same ID.

* Parse and utilize the unique_id field in the query parameters of the front-end view

- Handle the unique_id field when creating a new conversation from the home page
- Parse the id field with a lightweight parameter called v in the chat page
- Share page should not be affected, as it uses the public slug

* Fix suggested card category

src/khoj/database/models/__init__.py

@@ -350,6 +350,7 @@ class Conversation(BaseModel):
     title = models.CharField(max_length=200, default=None, null=True, blank=True)
     agent = models.ForeignKey(Agent, on_delete=models.SET_NULL, default=None, null=True, blank=True)
     file_filters = models.JSONField(default=list)
+    unique_id = models.UUIDField(default=uuid.uuid4, editable=False, unique=True)
 
 
 class PublicConversation(BaseModel):