[Task] Prevent duplicate header names for alert custom detail columns

[giranm]

Summary

When creating an alert custom detail column whose header already exists (e.g. Summary:details.title against default Summary column), the option is greyed out in the column selector. However, this workflow can be confusing as it implies it's a valid option that can be added to the view.

A preferred solution would involve a validation on the react-select input itself:

pd-live-react/src/components/SettingsModal/SettingsModalComponent.js

Line 353 in a042af7

<CreatableSelect

Ref: https://react-select.com/props#creatable-props