Adding a "spectrogram" poll type

[lidel (Marcin Rataj)]

To save some time: Catherine talks about spectrograms around 12:40

I think the backend is already in place (number poll with some arbitrary range, eg. 0-100).

The missing pieces:

• better input UI
  perhaps <input type="range" (..) could be used when rendering poll type=spectrogram? (example1, example2)
  this would make „fuzzy” voting between two options fast and intuitive

• additional spectrogram visualization of multiple datapoints