Remove the old Heroku slugignore file.
Convert to 2-spaces instead of 3-spaces.

It doesn't matter in practice, but I should probably follow the
standard Ruby format.
Remove assigned but unused error variable.
Rename the Assets controller.
Move the gist processor into a separate file.

This is the start of Ktty's future plugin system. I eventually want to
move plugins into their own folder, but this will do for now.