Make allowed Visiblity modes configurable for Users (#16271)

Now that #16069 is merged, some sites may wish to enforce that users are all public, limited or private, and/or disallow users from becoming private.

This PR adds functionality and settings to constrain a user's ability to change their visibility.

Co-authored-by: zeripath <art27@cantab.net>

routers/web/user/setting/profile.go

@@ -38,6 +38,7 @@ const (
 func Profile(ctx *context.Context) {
 	ctx.Data["Title"] = ctx.Tr("settings")
 	ctx.Data["PageIsSettingsProfile"] = true
+	ctx.Data["AllowedUserVisibilityModes"] = setting.Service.AllowedUserVisibilityModesSlice.ToVisibleTypeSlice()
 
 	ctx.HTML(http.StatusOK, tplSettingsProfile)
 }