Update tdwg.github.io/chrono/ to make use of new TDWG branding

[peterdesmet]

https://www.tdwg.org now makes us of Jekyll and the new TDWG branding (see tdwg/website#479). The Chronometric Age Extension website (https://tdwg.github.io/chrono/) does not and should be updated. Luckily it already makes use of Jekyll, so it is mainly changing theme and some layouts.

• Update _config.yml to e.g. make use of remote_theme: tdwg/petridish
• Move navigation to _data
• Verify/update frontmatter of all pages
• Test site locally
• Once dwc.tdwg.org, ac.tdwg.org and tdwg.github.com/chrono are updated, remove redirect for https://www.tdwg.org/theme/* (@MattBlissett)

I am happy to implement all this, but can't promise when.