Overhaul Stable Diffusion 1.4 web demo

[bgoelTT]

The current web demo's backend relies on reading & writing to a file (models/demos/wormhole/stable_diffusion/demo/web_demo/input_prompts.json) to facilitate the transmission of data between the flask server and the model backend process. This method makes no guarantee of mutual exclusion, sometimes leading to race conditions where the flask server is writing to the file while the model backend is reading, clobbering the file and crashing the server. Additionally, if multiple prompt submissions occur from e.g. 16 simulatenous users, the lack of mutual exclusion leads to undefined and error-prone behaviour.

This issue can be closed when a proper data-transmission method has been implemented, perhaps using python's queue.Queue which is thread-safe.