Fix wsgi_app - flag parsing & singleton

[wwwillchen]

This fixes two related issues fixed with our wsgi_app implementation:

• Previously, there was no flag parsing (for absl-py) when running Mesop via gunicorn (recommended for production serving) which resulted in a UnparsedFlagAccessError, because of this flag: https://github.com/google/mesop/blob/8ccc8d234f33d675fe3018c10817be917ee3a1a7/mesop/runtime/context.py#L15C1-L16C1. This uses the manual flag parsing method, instead of the usual app.run because app.run never returns, so we can't use it here.
• Previously, we were creating a new Flask app instance for each request, which technically works, but it's wasteful since a Flask app is supposed to be persistent. This saves ~5ish ms per request by caching the Flask app instance. This is also nice because then we can just parse the flags once too.