Composer action should be handling the parent field

[elboletaire]

The parent field has to be defined by the UI when doing the postMessage action to the warpcast app. Right now we're not handling it (the backend does not send it to the UI, and the UI does not define it in the postMessage), meaning if someone uses the composer action replying to someone (may be weird for our composer action, but the possibility's there), it will replace such reply with a new cast.