Firefox desktop page redirects for Australis

This PR covers the following redirect-related bugs for Australis and replaces #1803, #1811, #1928 and #1934. Some links are changed to match the context, instead of just being replaced.

* [Bug 979527 - [australis] redirect /firefox/central to a sumo page](https://bugzilla.mozilla.org/show_bug.cgi?id=979527)
* [Bug 979654 - [australis] redirect /firefox/security page to /firefox/trust](https://bugzilla.mozilla.org/show_bug.cgi?id=979654)
* [Bug 979660 - [australis] redirect /firefox/technology to MDN](https://bugzilla.mozilla.org/show_bug.cgi?id=979660)
* [Bug 979664 - [australis] redirect /firefox/performance to new Desktop Fast page](https://bugzilla.mozilla.org/show_bug.cgi?id=979664)
* [Bug 979670 - [australis] redirect /firefox/features to new Desktop overview page](https://bugzilla.mozilla.org/show_bug.cgi?id=979670)