Add fallback logic to ensure that uploaded context preserves references to CSS generated/referenced by LESS module

Problem/Motivation

Under normal operation, the smartling_translation_context submodule hooks css_alter in order to convert attached stylesheets into embedded style elements within the uploaded context document. When the LESS CSS Preprocessor module is also in use, the CSS array passed to the alter function may contain references to .less files which will be replaced in a later step just prior to page rendering. These references are ignored by the smartling module, and on a site where CSS aggregation is enabled, the resulting page will contain a mix of smartling-processed CSS and <link> elements generated by the LESS module processing phase. Those
elements are then stripped out by the htmlSaveComplete utility class of the smartling context module, which can result in the bulk of the theme styles not being present at all in the uploaded context. Needless to say this can severely degrade or eliminate the usefulness of the context.

Proposed resolution

As a general workaround, we propose a slight modification (see attached patch) to the offending class so that it preserves link elements which reference stylesheets. This should increase the likelihood that the uploaded context will display properly for certain sites where CSS processing may be subject to influences outside of the normal mechanism.