Restructure themes

This patchset:
- Moves all files for theme to one directory instead of having it scattered in img/, css/ and other directories.
- Makes per-theme mascot array (mascots.js), so themes can have mascots number and filenames independent from each other (and user don't have to specify them).
- Moves all variables which specify current theme in one place in index.html. That makes it easier to change themes than it were before (still not ideal, but I can't think of better way without complicating whole thing too much).

In present state there is one merge conflict - for this patchset to work properly, mine version of conflicting hunk has to be selected. It's straightforward resolution, so I'm leaving it up to you for the sake of having cleaner history.