Fix minor front-end issues with coupled must-run interconnectors

[louispt1]

Separate to #4341, because the interconnector coupling simply de-activates some sliders and replaces the upload module with the new coupling curves, we decided it's not clear enough that a coupling is in effect.

So this issue is to update the curves upload (when coupled) so that it also has the little coupling icon.

The icon would go on this part:

[github-actions]

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[kaskranenburgQ]

Some other issues concerning the external coupling of interconnectors we found are:

• Uploading a must-run curve returns the error message "An error occured". A refresh of the page indicates that the curve is actually loaded in properly, so the error message is false.
  
• When the external coupling input external_model_interconnectors is set to 1.0, all the slide texts for the curves will be set to "Coupled custom curves". Even when the interconnector is not coupled, as seen here: