fix(backend,parser): remove stopwords from normalized tags on node update

[perierc]

What

This PR fixes the fact that the stopwords were not removed during normalization of the tags outside the parser (i.e during node update, or node creation).

• Added an optional stopwords parameter (dict) to the normalize_text utils function, in order to remove stopwords from a given text.
• Modified the update_node controller to remove the stopwords during normalization thanks to the new parameter
• Refactored the parser to make it use the normalize_text utils function instead of its own method defined in its class

Part of

Fixes #367

[eric-nguyen-cs]

LGTM
I approved but please ask for another look if you decide to add a Pydantic model

[alexgarel]

Great :-)