Make search model configurable on server

- Expose ability to modify search model via Django admin interface
- Previously the bi_encoder and cross_encoder models to use were set
  in code
- Now it's user configurable but with a default config generated by
  default

tests/helpers.py

@@ -7,6 +7,7 @@ from database.models import (
     ChatModelOptions,
     OfflineChatProcessorConversationConfig,
     OpenAIProcessorConversationConfig,
+    SearchModel,
     UserConversationConfig,
     Conversation,
     Subscription,
@@ -71,6 +72,16 @@ class ConversationFactory(factory.django.DjangoModelFactory):
     user = factory.SubFactory(UserFactory)
 
 
+class SearchModelFactory(factory.django.DjangoModelFactory):
+    class Meta:
+        model = SearchModel
+
+    name = "default"
+    model_type = "text"
+    bi_encoder = "thenlper/gte-small"
+    cross_encoder = "cross-encoder/ms-marco-MiniLM-L-6-v2"
+
+
 class SubscriptionFactory(factory.django.DjangoModelFactory):
     class Meta:
         model = Subscription