Fix #580
fixed bug in HIDE_SOURCELINK/COPY_SOURCES logic
send unicode strings to slugify

Signed-off-by: Chris “Kwpolska” Warrick <kwpolska@gmail.com>
use utils.unicode_str instead

Signed-off-by: Chris “Kwpolska” Warrick <kwpolska@gmail.com>
Merge branch 'master' of github.com:ralsina/nikola into new_gallery_layout