✨ Add Custom to "Edit Nuisance Regressor"

[shnizzedy]

Related problem

A "Custom Regressors" section has been added to the nightly docs, but is not yet included in the GUI.

Additional context

"Custom" holds an array of objects

[
        {
            "file": "file containing the regressors. It can be a CSV file,
                with one regressor per column, or a Nifti image, with
                one regressor per voxel".
            "convolve": bool("perform the convolution operation of the given
                regressor with the timeseries").
        }
    ]

[sgiavasis]

Good catch.