fix(code): Fix invalid argument supplied for foreach() (refs #4486)

Fix PHP Warning: Invalid argument supplied for foreach() in
/var/www/c0/OFFLINE/Class.OfflineDomain.php on line 278
fix(code): Add JSON Content-Type to OFF_DOMAINAPI responses (refs #4486)

Also added a `html` HTTP var to trigger responses un plain HTML for debugging
purposes.
fix(CSS): Remove vendor-prefix and optimize CSS (refs #4486)
fix(code): Strip namespace in syncCodes (refs #4486)
fix(code): Correct PHP Notices (refs #4486)
fix(gettext): Correct po file's name (refs #3108)
fix(gettext): Update msgid declaration and regenerated po files (refs #3108)
Merge pull request #63 from eguaj/2.0-A4486

fix(code): Correct report generation (closes #4486)