fix: post channel reminders as custom message type

This fixes the "@channel" and "@here" keywords not triggering
mentions.

By default, bot users are not allowed to trigger channel mentions.
However, if the message type is not the default, this check is bypassed.

Fix scottleedavis#204

server/reminder.go

@@ -281,6 +281,7 @@ func (p *Plugin) TriggerReminders() {
 						ChannelId:     channel.Id,
 						PendingPostId: model.NewId() + ":" + fmt.Sprint(model.GetMillis()),
 						UserId:        p.remindUserId,
+						Type:          model.POST_CUSTOM_TYPE_PREFIX + "reminder",
 						Message:       T("reminder.message", messageParameters),
 						Props:         model.StringInterface{},
 					}