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

Edited Mar 28, 2021 by akkua98

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