R1:16d9cc12af45

Enforce viewable MIME types config on PDF documents

Summary:
Let instance admins decide whether to allow PDFs to be viewable as a Web page. See <https://github.com/mozilla-conduit/phabricator/commit/5ec132bf9ebfb90558f1b7f646772176629f86d0>.

MOZILLA: Instead of always allowing PDFs to be viewable in the web UI, [...]
This checks that the PDF mimetype is viewable according to the system
configuration.

Ref Q83.

Test Plan:
1. Set `files.viewable-mime-types` to exclude application/pdf.
2.