Add support for sending subject line text

[sabogalc]

The Android app can currently display subject text, but there is no way to send them due to the API limitations also discussed in issues #15 and #16. Below are some relevant links including documentation and similar feature requests for other apps.

https://docs.bluebubbles.app/private-api/imcore-documentation#sending-a-message-including-subject-effect-mention-and-or-reply https://github.com/sgtaziz/WebMessage/commit/6ad309540009680933536ae0cd50e125b670c9a1 https://github.com/open-imcore/imcore.react/issues/61