tailcfg: remove ancient UserProfiles.Roles field

And add omitempty to the ProfilePicURL too while here. Plenty
of users (and tagged devices) don't have profile pics.

Updates #14988

Change-Id: I6534bc14edb58fe1034d2d35ae2395f09fd7dd0d
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f