Ability to set message flags on post

[muddyfish]

The create message and send webhook endpoints should be able to have the flags field as an argument. This would allow us to send messages with embeds pre-suppressed, so we could repost user content and always be able to emulate their own 'embed links' permission. Currently if you want to do this, you need to make two requests, and the embed would pop out and then quickly vanish.

[advaith1]

#3236