This changes the public slug format from

31c3_-_6561_-_en_-_saal_1_-_201412271100_-_31c3_opening_event_-_erdgeist
_-_geraldine_de_bastion

to

31c3-6561-en-31C3_Opening_Event

a format similar to the filenames published in the recent years, but
without a language identifier. The original language is already
available in the schedule and can be appended if needed. Published
recordings may contain other languages as translations so the original
language should be excluded from the slug.

It uses truncate from TextHelper to enforce a maximum length. This
shouldn’t be an issue in general as event.title has the default string
column limit of 255 chars, but strange conference acronyms may break
that.

Calls to_str to get an unsafe string for HTML templates.