Feature themable buttons

One major feature of the whole scss refactoring is to be able to change the colors by setting only very few variables. While we added those variables some time ago they can still not be used to their fullest as buttons are not yet themable. This pull request tries to change that.

Buttons currently use background images. While most of them are simple gradients some contain additional icons and are not as easy to replace. I tried my best but it will look slightly different.

As the button styles are used in several places I converted most of the code to mixins that can be included anywhere in the code. When writing themes you can now simply overwrite those mixins and change the appearance of all button and button-like components (e.g. vote elements) consistently.

This PR also removes bootstrap-buttons and replaces them with our own.

Finally, I included two commits which depend on button mixins: One fixes buttons inside tutorials while the other applies button styling to the subnavigation. This will, too, look different from before.

I am very open to proposals and changes to my code. This is the last big step in the scss refactoring so I want to finish this so I can finally use the new features for theming.