Dynamically select issue labels

[mrousavy]

It would be awesome to dynamically assign people and select issue labels depending on user choices.

For example, if there is a dropdown which selects the Platform:

and the user selects iOS I want to apply the iOS label to the issue. This could potentially also apply to assigning different people depending on the user's selection.

[lukehefson]

Please see: #4266 ❤️