show uploading files in conversation

[mahibi]

uploading files should be shown in chat, including a progress indicator and a cancel button.

right now for files >1MB there is a notification with progress, for files <1MB there is no notification. for both big and small files, there should be a preview message in the chat while uploading. maybe it makes sense to postpone this until https://github.com/nextcloud/talk-android/issues/1552 is done.

[enakhala]

1mb file are failing to upload with V18.0.1