This adds wiki page front-matter parsing library code, behind a
feature flag. The parser abstracts front-matter recognition and
extraction and explanation of failure.

The WikiPage model is provided with a new attribute `front_matter`,
which is derived from the content. This attribute can be updated and
serialized to the wiki-page repository.

The feature flag is gated on a project-by-project basis.

Front-matter parsing is unified across concerns, by
re-using our existing front-matter parsing Regexp in the new
front-matter parser, to ensure consistency.

A new module, `Gitlab::WikiPages` is used by model and library code.

`parse_string` is made a separate method so that different languages can
be supported without modifying other code.