Creating an environment can take a long time and seem unresponsive

[matthewelwell]

Currently, when creating a new environment, it is only possible to do so by cloning from an existing environment. This process can take a long time, so the UI often looks like it's hanging.

There is a PR open on the BE here which asyncs this logic and adds a new is_creating status to the environment which the FE should use to show a UI prompt, and prevent the user from opening the environment until the creation is complete.

[matthewelwell]

Instead of making the BE automatically async the cloning of feature states, I have added a new attribute ("clone_feature_states_async": true|false) to the payload on the clone environment endpoint to trigger the async behaviour. Note that the default is false meaning that we should be able to merge the BE without needing the FE changes.