Allow users to magnify beyond 200% and maintain function of the app (PDFs)

This issue is part of our effort to become WCAG 2.0 AA compliant.

1.4.4: Resize text
- [ ] Allow users to magnify beyond 200% and maintain function of the app (A)

Problem:
Users should be able to increase the size of text to 200% without loss of content or
functionality and without requiring horizontal scrolling. This works well for HTML documents, but doesn't work for PDFs.

Steps to reproduce:
- Activate Hypothesis over a PDF - [This one, for example](https://d242fdlp0qlcia.cloudfront.net/uploads/2018/05/25160432/HypothesisWCAGReview20180514.pdf).
- Using your keyboard, attempt to magnify your browser tab by 200%

Expected results:
Like with an HTML page, both the document and sidebar are magnified

Actual results:
The PDF content is magnified, but the sidebar is not:

Compare this to the HTML experience:

Notes:
See W3C guidelines for scale/magnification here: https://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio-contrast-scale.html