Integrate attachment functionality in new ComposeView

Currently in the newly redesigned ComposeView it is not possible to add and send attachments. Add UI and functionality for attachments. Desired behaviours include:

• add floating action button to enable adding attachments
• let attachments section slide from the bottom
• adding files as attachments
• removing files from attachments
• previewing attachments
• open attachments full-screen conveniently
• remind to add attachments if mentioned in E-Mail