Add Volunteer role to all members - discord ID

[joepio]

When new Members join, we want to give them the Volunteer role on discord. We ask for their Discord username.

However, I think we need their id as that's the unique identifier! So that requires an update to the form... And asking users for their ID is maybe a bit tricky. It requires them to press the profile in the bottom left, and press Copy User ID.

[Wituareard]

We could fetch the member list and filter by username, but that would require the "member list" intent to be enabled for the bot account.