[br] Allow '/' in URL, allow empty author + broadcastDate fields

* Allow URLs that have a 'subdirectory' before the actual program name, e.g.
  'xyz/xyz-episode-1'.
* The author and broadcastDate fields in the XML file may be empty.
* Add test case for the two problems above.
Merge remote-tracking branch 'soult/br'
[vevo] Centralize timestamp handling
[ninegag] Fix extraction
[googlesearch] Fix next page indicator check
[wdr] Add description to tests
[videobam] Fix empty title handling
[test_download] Move assertions before debugging output
[udemy] Initial support for free courses (#1617)
Rename upload_timestamp to timestamp
[vevo] Adapt test to constantly changing timestamp
Add bestvideo and worstvideo to special format names (#2163)