Consolidate Create Model actions into dropdown button for Pajamas compliance

Overview

We need to update the Create Model and ellipsis buttons - both of which currently house creation actions - to be a single, primary dropdown button containing both actions (Pajamas guidelines for this can be found here). The default action (button text) should be 'Create new model', and the dropdown action should be 'Create with MLFlow' as seen below:

Current experience	Desired outcome