MultistageChannelFilter sanity checks cutoff_freq.
Eliminate 'config' arg to WebService.
Separate DB reading from web resources.
Allow multiple services.
Move code assuming services have urls into the web service.
Listen to slider 'input' events to capture all updates.

HTML5 spec says that the 'change' event isn't fired until the change
is 'committed', which for a slider is letting go. This was not
implemented as specified in Chrome until recently, so we didn't notice.
Quiet pylint some more.