[BUGFIX] Fixing external ListVariable overriding dashboard ListVariable in query param

[Gladorme]

Description

Doing a quick fix for issue #1954, I think there's still room for improvements. Lot of useEffects to remove in TemplateVariable.tsx + we should have a method returning all variables def a dashboard can use and remove variables definitions overrode. I will check when I have time 😄

fixes #1954

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[celian-garcia]

I'm interested on that fix, can we call each other in 30min ?