Add Open Tree of Life resolution

[gaurav]

This PR adds a "Add Open Tree of Life phylogeny" button to the sidebar, which adds the induced subtree from the Open Tree of Life that includes all named taxa used as specifiers across all phyloreferences. Taxa that don't have OTT IDs are ignored, while OTT IDs that not included in the synthetic are ignored. Closes #40.

This appears as a button in the bottom left corner of the screen labeled "Add Open Tree of Life phylogeny":

Clicking this button doesn't have any obvious signs that anything is happening (I'm not sure where they would go), but once the API calls have completed, a new phylogeny labeled "Open Tree of Life phylogeny" has been added to the Phyx file:

This phylogeny is the induced subtree of all phylogeny nodes that have Open Tree of Life Taxonomy IDs:

(Please ignore the "Primary Reference phylogeny" field; once we close #133, only the "Reference phylogeny" field will remain, and it'll be renamed to something like "Citation").

With the changes I made in 26f836e, this now includes a citation to the Open Tree of Life synthetic phylogeny, as suggested at https://tree.opentreeoflife.org/about/open-tree-of-life -- I've included both the synthetic tree and taxonomy version numbers as reported by the Open Tree API, but only cited the synthetic tree, since we only use the taxonomy to look up nodes on the synthetic tree.

[gaurav]

TODO:

• Add a screenshot to this PR showing how this is to be used.
• Remove internal nodes that only have a single child, as these just clutter up the user interface for no reason --> moved to Add "Simplify phylogeny" button to simplify trees (especially Open Tree of Life induced subtree) #241

[hlapp]

Looks good. See one inline suggestion.