fix: Addded flask-cors for web plugin to use

[matlads]

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• [x] I have read the contributing guideline and understand that I have made the correct modifications

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Description:

Add flask-cors python package to the image.

Benefits of this PR and context:

This PR adds to the python packages available to the app. The web plugin uses flask-cors

How Has This Been Tested?

1. Bulit a new docker image
2. Run it and changed the config and added cors support to the web plugin.
3. Used an external beet web frontend (beetle) to play a track.
4. No changes to any code

Source / References:

web plugin documentation