Add more filters for spec. stability

[SebastianZ]

At the moment there are two filter options "Stable" and "Experimental" that refer to the specification stability. "Stable" refers to any specification that is mentioned within the CSS snapshots.

In #223 I asked how filtering by stability should work and in #220 @PolariTOON and I agreed that it makes sense to add explicit filters for specifications that are marked as "roughly interoperable" and "fairly stable" in the CSS snapshots.

One open question here is how the UI for those filters should look like. @PolariTOON's suggestion for that was:

"fairly stable" and "roughly interoperable" could have their own filter choice / checkbox next to the snapshots, but perhaps we should only make them according to the latest snapshot to avoid having some specs appear in several filters. Since we already use to test these modules in this project, just adding a way to filter them should not have a big cost.

The idea behind that was to turn the filter drop-down list into a group of checkboxes and add two for the two stability options.

As a first step into a more fine grained filtering system, I imagine to display those two checkboxes when a CSS snapshot is selected.

Sebastian