Remove conference / event names from tags

[jonafato]

Conferences, events, and other collections of videos are the primary types of collections supported by PyVideo, so these should be supported as high-level structured data (i.e. in category.json) and not be included as tags. A good example of this is "PyCon", which may refer to a number of different Python conferences. If applicable, redirects may be added for tags that do refer to a specific event series (e.g. "pyohio"), but this may not always be possible.