Use new flatpages meta parser

[lukasjuhrich]

v0.8.0 of flask_flatpages introduced a new meta parser.

For instance, parsing of content/news/2016-09-01-upcoming-office-hours.en.md fails.
This might be considered a bug on the flask_flatpages side. We should find the cause of the parsing error and/or report it.

As a workaround, we currently set FLATPAGES_LEGACY_META_PARSER to True.

[lukasjuhrich]

See https://github.com/Flask-FlatPages/Flask-FlatPages/releases/tag/v0.8.0