Option to not include webhook_data to reduce message size

[moorglade]

I use Matrix Hookshot for processing alert notifications from Alertmanager. Such notifications can sometimes get pretty large (over 64kB). In such cases, sending a message through Hookshot fails with the following error:

{ errcode: 'M_TOO_LARGE', error: 'event too large' }

I prepared a JS transformation function which reduces the message size, but this does not help. I guess this is because Hookshot attaches the original message to the request body in the uk.half-shot.hookshot.webhook_data field.

An option to not include uk.half-shot.hookshot.webhook_data in the request sent to Matrix would be very helpful in my use case. Or maybe there is some other way of handling large messages?

Possibly related to #613