Intent to Unship: <a rel="sidebar"> support

[Mark Banner]

In bug 1452645 [1] we are planning to remove the "Load this bookmark in
sidebar" feature. As part of this we will also be removing support for
rel="sidebar" in anchor elements.

In the future, anchor elements with rel="sidebar" will be treated like
normal anchor elements.

We are planning to make the change in Firefox 63.

From the specifications side, rel="sidebar" is only implemented by
Firefox [2] and although it seems to have been mentioned in early
versions of the HTML 5 specification, it isn't in the final version.

Extensions are able to open pages in the sidebar [3] (e.g. side view
[4]), and intercept loads (e.g. see the Facebook Container add-on [5]),
hence, they could add very similar functionality.

[1] https://bugzilla.mozilla.org/show_bug.cgi?id=1452645

[2] https://developer.mozilla.org/docs/Web/HTML/Link_types

[3]
https://developer.mozilla.org/Add-ons/WebExtensions/API/sidebarAction/setPanel

[4] https://testpilot.firefox.com/experiments/side-view

[5] https://addons.mozilla.org/firefox/addon/facebook-container/