fixed some bugs with app icons; removed extraneous files

this fixes a few things:

1. the default app icons were SVGs, which don't render in some older browsers (notably <= IE 8). This changes them to PNGs.
2. the icons in the picker would be highlighted black when hovered over, which would make them totally black due to icon transparency. This changes the highlight color to a light pink in line with our color scheme.
3. 'http://' and the 'thisnode' alias were prepended to each app icon path unnecessarily. This removes that, and has the benefit of not breaking if we decided to change the name of our alias in the future.
4. Removed lots of extraneous files in the /luci-static directory.

To test:
1. on the create app page, and mouse hover over the default icons
2. choose one of the icons, and submit the app. make sure it shows up correctly on the listing.
3. Edit the app, and change the icon to be an external image on the internet. Make sure it shows up correctly on the listing page (might need to get splashed so it allows external requests).