Work around a Chrome bug [1] that can cause the sidebar to become
invisible if:

 1. The sidebar app is loaded from a Chrome extension AND
 2. The current tab was opened by clicking a link inside the sidebar
    app in a different tab.

When the issue occurs, the sidebar web app loads and runs normally but
is just not visible on screen. This happens due to an internal issue in
Chrome which can be avoided adding `rel="noopener"` to all "normal" [2]
links in the client that open URLs in a new tab/window.

Doing so enables Chrome to use a separate process for the Hypothesis
client in the new tab in step (2) than the one used for the Hypothesis
client in step (1). This change also prevents potential tab-jacking
attacks in all browsers that support `rel="noopener"`.

Fixes #516

[1] https://bugs.chromium.org/p/chromium/issues/detail?id=753314
[2] ie. Those which do not use JS to handle the link