A basic way to convert normal users to other types (just PROXY_USER for now)

[IRHM]

Idea Overview

We need a simple way for an admin to convert the type of a users account.

Is your feature request related to a problem? Please describe.

With the addition of trusted header auth (#736), existing users wanting to use this new auth process have no way of migrating their account to the PROXY_USER type.

Describe the solution you'd like
Ideally an admin can quickly swap around user types in the /users/ dash.

For now, we can leave it at support for changing between a normal user and a proxy user (since this is the simplest, other user types (i think) would require additional config when migrating the user).