handle encrypted and unencrypted events
get the correct event_id (the base message) when called from EditHistoryMessage
keep only the `body` and `msgtype` fields when prefilling
reuse component SendCustomEvent
swap it in place in the View Source dialog
the Back button takes you to the View Source dialog, not the DevTools dialog
do not display the flip toggle box for changing between State Event and Normal Event
We use `export default` begrudgingly here. Ideally we'd use just `export`, though this entire SDK expects things to be exported as a default. Instead of breaking everything, we'll sacrifice our export pattern for a smaller diff - a later commit can always do the default export -> regular export conversion.
