Re-index learned directories when embedding provider changes

[3coins]

Problem

When embedding provider is updated, existing vector store index does not work because of the embeddings mismatch. This causes an assertion error when /ask command is used and require users to manually delete the index with the /learn -d command.

Solution

1. When the embedding provider changes, indicate to the user that doing so will cause issues with the existing index, and the existing index will be deleted.
2. Prompt the user if they want to still go ahead and they also want to index all the directories they have indexed before.
3. Update the backend to delete the existing index, and re-index all the directories that have been learned before.

[3coins]

Completed with #137