Instead of just upstreaming the changes from [bug 1345253](https://bugzilla.mozilla.org/show_bug.cgi?id=1345253) as-is, it seemed better to simply get rid of the loops altogether and use the same approach as in `PDFViewer`/`PDFThumbnailViewer`.
If users want to download, they can quickly click on the Download button
in the newly opened viewer.
The blobUrl logic for Firefox relies on `disableCreateObjectURL` is
never false in Firefox. If the assumption is invalid, then PDF
attachments at the attachment view will not correctly be displayed,
because a data-URL will be generated and `?<filename>` is treated as
part of the data:-URL.
Other PDF viewers, e.g. Adobe Reader, seem to append `FileAttachment`s to their attachments views.
One obvious difference in PDF.js is that we cannot append all the annotations on document load, since that would require parsing *every* page. Despite that, it still seems like a good idea to add `FileAttachment`s, since it's thus possible to access all the various types of attachments from a single place.
*Note:* With the previous patch we display a notification when a `FileAttachment` is added to the sidebar, which thus makes appending the contents of these annotations to the sidebar slightly more visible/useful.
Changes `PDFOutlineView`/`PDFAttachmentView` to be initialized once, since we're always creating them, and refactor their `render` methods to instead pass in the `outline`/`attachments`.
For consistency with other "classes", the `PDFOutlineView`/`PDFAttachmentView` are renamed to `PDFOutlineViewer`/`PDFAttachmentViewer`.
Also, make sure that the outline/attachments are reset when the document is closed. Currently we keep the old ones around until the `getOutline`/`getAttachments` API calls are resolved for a new document.
