Show list of active downloads

[mathebox]

Is your feature request related to a problem? Please describe.
After starting the download of learning material (video, slides, documents), the download progress is only shown next to the button for starting the download (content lists and detail pages). However, there is no view aggregating all active downloads across all courses.

Describe the solution you'd like
Add a cell titled Active Downloads to the Downloaded Content view in the Account tab. Tapping on this reveals a list of all currently active downloads grouped by course (title). Each cell should contain at least the following information:

• title
• progress
• stop/cancel button
• (icon for) download type (video, slides, document)

Affected Component
DownloadedContentListViewController

Additional context
The Active Downloads cell should only be shown if there are active downloads. The list of active downloads should update along with changes to the download state. For this, it is required to listen to the following notifications:

• DownloadState.didChangeNotification
• DownloadProgress.didChangeNotification
  (see VideoViewController for usage)