Currently in the newly redesigned `ComposeView` it is not possible to add and send attachments. Add UI and functionality for attachments. Desired behaviours include: - [x] add floating action button to enable adding attachments - [x] let attachments section slide from the bottom - [x] adding files as attachments - [x] removing files from attachments - [x] previewing attachments - [x] open attachments full-screen conveniently - [x] remind to add attachments if mentioned in E-Mail