page meta links should respect `.Site.IsMultiLingual`

[tmc]

Currently, the page meta links will always include "en" even if the site is not multi-lingual and has content directly in the "contents" directory (vs "contents/en").

[LisaFC]

This is a problem that has come up before (see #138) and we haven't found anything elegant that worked with the templating. Hopefully now that we have a bigger community (and possibly Hugo has changed a bit) we can come up with a solution.

Duplicate of #138