Deleting an agent attempts to delete any Taxon trees they creating

[bronwyncombs]

Describe the bug
A clear and concise description of what the bug is.

For a specifyUser that has created a taxon tree (the only type of tree able to be made by a user).

Deleting the agent associated with their specifyUser will delete the taxon tree(s) created with warning. This is not caught by delete blockers.

specify7/specifyweb/specify/models.py

Line 7303 in fb1c6d6

createdbyagent = models.ForeignKey('Agent', db_column='CreatedByAgentID', related_name='+', null=True, on_delete=models.CASCADE)

To Reproduce
Steps to reproduce the behavior:

1. Create taxon tree in Specify
2. Note the agent associated with the current user then log out
3. Log in with a different user
4. Delete the agent associated with the user you created the tree as
5. Go to trees > taxon
6. See the taxon tree the agent/user made is gone

Please fill out the following information manually:

• OS: macOS
• Browser: chrome
• Specify 7 Version: 7.12.0.7
• Database Name: kufish
• Collection name: ku fish tissue
• User Name: spfishadmin
• URL: [e.g. https://ku_fish-v7.test.specifysystems.org/specify/workbench/215/]

Reported By
Jason