Fixed some (pdf) template issues and updated (DE) translation.

- Fixed #737 (Motion edit link is missing for normal delegate users)
- Fixed motion pdf filename errors (now use motion-<identifier>.pdf or if no identifier: motion-<title>.pdf with using djangos slugify for special chars in title)
- Fixed motion pdf build error if there is an unauthorized version
- Enlarge max_length of event name field to 50 chars.
- Align participant import instruction with motion import page.
- Added title field to particpants-demo_en.csv.
- participant overview: Show comma after last_name only if first_name and last_name are given.
- Changed all page titles to "<specific action page> - <event-name>" (moved event-name to the end of the string)
- Updated EN tranlation string.
- Updated DE translations.