Switch away from using nevow

[habnabit]

As part of the effort of porting to python 3, moving the web code away from nevow is the simplest option, since nevow won't be updated (and the code is using old parts of nevow anyway) but twisted.web is already compatible with python 3. twisted.web.template already does most of what the templates used in nevow in the first place.

Since nevow and twisted.web interoperate enough, this port can be done piecemeal at the granularity of one template per ticket/PR.