[WIP] Add toggle to desktop to flip compatibility filtering (bug 969244)

https://bugzilla.mozilla.org/show_bug.cgi?id=969244

TODO:
- [ ] Hide the checkboxes for Android
- [ ] Add UI tests
- [ ] Tweak design depending on feedback in the bug

Everybody loves screenshots:

When logged out, you don't see "Sign in" anymore, only the gear icon.
![gear_icon_logged_out](https://cloud.githubusercontent.com/assets/187006/4013070/3f5accd0-2a13-11e4-9566-d1d7a6fee7a3.png)

Hovering over it reveals the dropdown for anonymous users:
![dropdown](https://cloud.githubusercontent.com/assets/187006/4013087/61eea294-2a13-11e4-997a-d47e7851c7d4.png)

What you see if you hover "All Devices" radio button (the view is reloaded when you click, changing the results directly without closing the dropdown):
![all_devices_hovered](https://cloud.githubusercontent.com/assets/187006/4013091/6b7a916a-2a13-11e4-9c42-6c675d1c54cc.png)

What it looks like when you are logged in:
![dropdown_logged_in](https://cloud.githubusercontent.com/assets/187006/4013097/8797226e-2a13-11e4-82b4-23603ab0c67c.png)

(Some of the screenshots have the semi-transparent scrollbar from Firefox Responsive Mode on the right, some others have a black border on the left because I suck at GIMP, it's not a visual artefact caused by the CSS, I swear :)