Prevent adding children to synonym nodes when not authorized

[CarolineDenis]

Fixes #4868

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

Testing instructions

Note: Remote Preferences should not have the line: sp7.allow_adding_child_to_synonymized_parent.TREENAME=true
If it does, set it =false or delete the line, then clear your cache.

• Go to a Synonymized node in a tree (ex: testchild-syn on Taxon https://umherb11024-edge.test.specifysystems.org/specify/tree/taxon/?conformation=%7E%7E1%7E573177%7E573193----)
• Click on Edit -> Add
• Input the synonymized node's name into Parent field, then fill in required fields (ex: testchild-syn; test)
• verify that a tree structure warning is shown and save is disabled

[emenslin]

Testing instructions

Note: Remote Preferences should not have the line: sp7.allow_adding_child_to_synonymized_parent.TREENAME=true
If it does, set it =false or delete the line, then clear your cache.

• Go to a Synonymized node in a tree (ex: testchild-syn on Taxon https://umherb11024-edge.test.specifysystems.org/specify/tree/taxon/?conformation=%7E%7E1%7E573177%7E573193----)
• Click on Edit -> Add
• Input the synonymized node's name into Parent field, then fill in required fields (ex: testchild-syn; test)
• verify that a tree structure warning is shown and save is disabled

Looks good, cannot add a child to a synonymized node

[combs-a]

Testing Instructions

• Go to a Synonymized node in a tree (ex: testchild-syn on Taxon https://umherb11024-edge.test.specifysystems.org/specify/tree/taxon/?conformation=%7E%7E1%7E573177%7E573193----)
• Click on Edit -> Add
• Input the synonymized node's name into Parent field, then fill in required fields (ex: testchild-syn; test)
• verify that a tree structure warning is shown and save is disabled

13_synchildtest.mp4

Looks good, blocked it correctly. Good work!