Temporarily enable entity loader when importing XML file.

Similar to https://github.com/gothfox/Tiny-Tiny-RSS/commit/3457ce7c5963e507a2116866cedb1f1509230c93 as detailed in [this thread](http://tt-rss.org/forum/viewtopic.php?f=1&t=3231), this temporarily enables the entity loader when importing archived and starred articles from another TT-RSS instance.