fix #1075 -- use self.config['COPY_SOURCES']

Without this:

 * HIDE_SOURCELINK and COPY_SOURCES are unset, so
 * Nikola.config fills them as (False, True), but
 * Nikola.__init__ checks conf.py, which results in
 * HIDE_SOURCELINK = True, but COPY_SOURCES = True, which
 * works incorrectly and is unintended, and does
 * exactly what this was meant to protect against.

(commit message v2, clause 4 was wrong)

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