redesign v1

jira issue: HUBHAC-10
(you can see screenshots of the proposed changes in this jira issue)

let's treat this PR as a first step, as I don't want to block the work, and I have webinos work time limits, I'm afraid...

what it does:
- html cleanup
- separately scrollable areas with custom scrollbars (check tinyscrollbar_update())

what it doesn't do:
- settings popup, gauges, graphs are not styled yet, icons for them would probably need some kind of redesign too
- I've added a "drop here" indicator, this is buggy right now, but adding sensors was like that even before that - and that needs some more love (in some cases cut the user before he can perform the drag action instead of always showing the alert with error)