Zotero integration

### Problem you are trying to address

Users who use [Zotero](https://www.zotero.org/) expect Hypothesis to work on PDFs stored in Zotero.

@RaoOfPhysics (originally in https://github.com/hypothesis/h/issues/4362):

> ### Steps to reproduce
> 1. Add PDF to Zotero library.
> 2. With Zotero settings configured to open PDFs within the browser, double-click on PDF in Zotero add-on within Firefox to launch PDF in a separate tab.
> 3. Use JS bookmarklet to load hypothesis layer.
>
> ### Expected behaviour
> I would expect the hypothesis sidebar to appear upon clicking the bookmarklet.
>
> ### Actual behaviour
> The sidebar doesn't appear. However, hypothesis itself loads -- the buttons for "highlight" and "annotate" appear when selecting text in the PDF. But I cannot actually annotate the text itself (although highlights seem to persist).
>
> ### Browser/system information
> - Firefox 51.0.1
> - macOS 10.12.3
> - Zotero add-on 4.0.29.16
>
> ### Additional details
> The issue only appears with PDFs opened via Zotero, which uses `zotero://` in the URL bar to identify the PDF. If I open the same PDF using the "Open file" command (i.e., with `file://` in the URL bar), hypothesis works perfectly -- see screenshots to compare the two.
>
> - Using `zotero://`: Buttons appear, but I cannot annotate. Sidebar is missing.
>
>
> - Using `file://`: Hypothesis fully loaded.
>