[Task] Restore the method to retry sending of failed messages

[pixlwave]

Part of the crypto work from element-hq/element-meta#2488

Whilst most send failures can be resolved automatically (e.g. network failures, invalid access token etc) or are deemed unrecoverable (e.g. invalid data, too large etc) there are some instances where a message can fail to send but may work later:

• The other user was verified but has become unverified (will succeed when they're verified again).
• The other user is verified but has an unverified device (will succeed when the device has been verified).

Having a method to retry sending of an item will be useful for this.

[bnjbvr]

As the current timeline maintainer, I would consider this to be a regression, as the intent in terms of design is that the Timeline API is only a view onto the timeline events. Right now, this is not the case, but the overall effort of moving things out of the timeline into the send queue or into the event cache or other APIs would be wasted, if we restored that method to retry sending in the timeline itself. Let's not do this; can you think of an alternate way to do this?

[pixlwave]

My bad, I shouldn't have specifically said the timeline.

As long as it exists on the room/send queue/timeline item/wherever that would be enough to allow the user to resend a message.