Fix KeyError exception when flask-babel is installed but not used

[cjmayo]

flask-babel's gettext() expects a Babel object to have been created.

Otherwise an exception is raised.

  File "/usr/lib/python3.12/site-packages/flask_babel/__init__.py", line 46, in get_babel
    return app.extensions['babel']
           ~~~~~~~~~~~~~~^^^^^^^^^
KeyError: 'babel'

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Not completely convinced by this, but at least it illustrates the issue.

[cjmayo]

Indeed, it breaks applications where flask-babel is used.

[samuelhwilliams]

Yeah I've been chewing over this one for a little while in the back of my head. It's a bit gnarly because it mutates the entire flask_admin.babel module based on what's in place at the time of the first import.

I think it would be better to refactor this so that the state of the flask app, flask_admin instance, etc aren't relevant at module import time. But haven't yet dug into this enough to work out what that work look like and/or how far the repercussions would spread.