Consistency & Clean up

[Kriesse]

@laurengarcia A few things I want to walk through:

• [x] use the same scss import method everywhere
• [x] does hyperdrive-ui need css?
• [x] remove overly granular modules (dat-button, dat-header) (still needs to be done in dat-desktop)
• [x] use dat-design on dat documents (http://docs.dat-data.com/)
• [x] replace less with scss on dat-data.com? so we can use dat-design also?
• [x] icons (https://github.com/datproject/design/issues/12)
• [x] agree on css class structure / naming convention and document it in dat-design

[Kriesse]

replaced less with scss on website: https://github.com/datproject/website/pull/45

[Kriesse]

Done; remaining inconsistencies are tracked here https://github.com/juliangruber/dat-desktop/milestone/1