Use Flask application factory

[Ariana-B]

Rather than creating the Flask app module-level across _model.py and _pages.py, use the preferred application factory pattern to instantiate.
This highlighted an issue around how the default timezone is determined. Having CUBEDASH_DEFAULT_TIMEZONE be provided via both an environment variable and the app config seems like overkill, and which version is used - as well as the default value - appears to be inconsistent. Timezone configuration would likely be best revisited and properly reworked in another PR, but for now I suggest having it configurable only via the app config.

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📚 Documentation preview 📚: https://datacube-explorer--614.org.readthedocs.build/en/614/

[whatnick]

Please fix merge conflicts, not sure why the CI has paused.

[omad]

Looking good, I think one thing to change now, and some more for later (because they're harder, and this is still a big improvement).