This is a follow-up to edx#1087, which reverted this change. According to the PR comments, parsing strings with XML comments inside them was causing errors. This does not seem to be the case anymore - these strings are just hidden when the block is rendered, but they are not breaking XBlocks. This also handles (ignores) the comments that could be added directly to the LibraryContentBlock in the XML export by users.
common ------ This directory contains common code shared between LMS and CMS, such as Mako templates, CSS, and Coffescript.