add synth_id option to "about" to retrieve metadata for old versions

[arlin]

This is discussed in #222. Currently the information is available here https://tree.opentreeoflife.org/about/synthesis-release . It might be helpful to make it available via the API of tree_of_life/about, e.g., add a synth_version argument that takes a synth_id and returns the metadata for that version.

This is not a critical use-case for Phylotastic. For us, the problem is mostly solved by having this information available interactively via the web site.