Add admin function to initiate user password reset

Add admin function to initiate user password reset

This will trigger the same reset-your-password email as a user initiated
one, but it'll cut out one step and be a bit more user friendly...

Also, if this is done with an OAuth connected account, it will be
converted into a regular one (something we don't allow the end user to
do, for support reasons)

This also adds an entry to the user editor in the admin view that shows
if the user *is* an oauth user or not, or if they might have an old
"unmigrated" password.