Remove "CSS" from project titles and page names/URLs

[emalasky]

CSS Shapes -> Shapes
CSS Masking -> Masking & Clipping
CSS Regions -> Regions

[jacobgoldstein]

This needs to be done before others start working on bug fixing since it may affect page names and URLs

[mvujovic]

@emalasky I think it's typically "Clipping & Masking" instead of "Masking & Clipping"- I'll change to that unless there's any objections.

[mvujovic]

This is done. Let me know if I've missed anything.

I haven't updated the content of pages themselves, including the subtitles because I'm not sure it makes sense for all of articles- I think the content author should choose when to use "CSS" and when not. I've filed #74 to cover that.