This was used when you pressed the "Attach file/image" button but had an
image copied to your clipboard - allowing you to select from either
source. This is a weird thing to ask, the button should always prompt
you with a file dialog.
It's still possible to paste an image from your clipboard with CTRL+V,
but there isn't a way to do it via right-click yet.
Fixes#712
Move reply pane to use MessageContentModel. This means the reply pane component is no longer required.
This commit also limits the size of code and image componets in a reply to keep them from getting too huge.
