add alias exts in COMPILERS for "translated" content without a def. lang post

when somebody had the old TRANSLATIONS_PATTERN="{path}.{ext}.{lang}"
DEFAULT_LANG="en"
and a post "something.rst.es" and no "something.rst", the former
would be considered a file with wrong extension
this commit adds aliases like ".rst.es" to the COMPILERS at runtime
move extension lang-alias creation code to Nikola.__init__

to create the aliases only once and not in each function call
flake8 style fixes
more flake8 style fixes
fix COMPILERS exts alias creation

splitext assumed the ext was a apth, not ext, so a small path
"f" had to be added first
fix recongnition of files with a longer ext

splitext would only get the shortest ext, so endswith() has to be used
add a test case for posts only in secondary languages.

this test however only checks that nikola runs and creates something
it cannot check that titles are translated (as we remove the
translation)
flake8 fixes

remove whitespace and do not redefine ext in list comp
Merge pull request #1016 from getnikola/fix_non_translated_posts

Fix "translated" posts without original in default lang