backup projects: report warning if not everything could be written to backup

[bs-jokri]

From @heydenreich on December 2, 2016 11:36

When downloading a project backup archive, not all linked documents might be exported because of limited visibility-permissions of the user. The user should see a warning about that.

Challenge: return a possibly large .zip-file (including attachments!) in combination with metadata (e.g. RequestSummary including RequestStatus, number of totally treated docs and number of successfully treated docs). The limit is 500MB when doing this via Javascript in Firefox. Therefore, we have to find a different solution, e.g. presenting the metadata and a download link to the user and storing the zip-archive somewhere for later access via the download link.

Copied from original issue: bsinno/sw360#361