Add a button to download default trees

[CarolineDenis]

Fixes #6123

Checklist

• Self-review the PR after opening it to make sure the changes look good and
  self-explanatory (or properly documented)
• Add relevant issue to release milestone
• Add relevant documentation (Tester - Dev)

Testing instructions

New icon introduced:

• Open the tree viewer for Taxon
• Verify that there is no Download New Tree icon under the first rank if the tree has nodes (other than root)
• Verify that there is the Download New Tree icon under the + Icon (Create Root Node) in the first rank column for new empty trees
• Verify that there is the Download New Tree icon under Root in the first rank column for trees that only have a root node
• Verify that clicking the new icon opens https://files.specifysoftware.org/taxonfiles/ with a list of default trees to download from

TODO:

• Add explanation in https://files.specifysoftware.org/taxonfiles/ for what to do with the dowloaded tree

[lexiclevenger]

Testing instructions

• Open the tree viewer for Taxon
• Verify that there is no Download New Tree icon under the first rank if the tree has nodes (other than root)
• Verify that there is the Download New Tree icon under the + Icon (Create Root Node) in the first rank column for new empty trees
• Verify that there is the Download New Tree icon under Root in the first rank column for trees that only have a root node
• Verify that clicking the new icon opens https://files.specifysoftware.org/taxonfiles/ with a list of default trees to download from

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Looks good! One thing I noticed--when starting from an empty tree or one with a root node, the download button stays after you add nodes until the page is refreshed. Is this acceptable?