Don't filter for ancestors if target is root node

[bjester]

Description

This issue was caused by a pending call to get the ancestors of the root node of the channel, which blocked the loading of the page and thereby prevented copying. Unfortunately, this ancestors call can take several seconds on hotfixes and production (90 seconds on hotfixes for me once), even though the root node has no ancestors. This PR makes a change to take a shortcut in that scenario to only return the target node.

Issue Addressed (if applicable)

Addresses #2927
Addresses #2889

Steps to Reproduce/Test

• Create a new channel on hotfixes
• Open the channel
• While the ancestors call is pending, you should see an empty gray channel
• Copying is not possible until the ancestors call completes and you see "Click ADD..."
• Once the API call completes, which should be much faster with this change, you should see the screenshot below and should be able to drag and drop from clipboard to the channel
  

Checklist

• Is the code clean and well-commented?
• Are there tests for this change?

[codecov]

Codecov Report

Merging #2992 (5c2410f) into hotfixes (28587f8) will increase coverage by 4.99%.
The diff coverage is 92.92%.

@@             Coverage Diff              @@
##           hotfixes    #2992      +/-   ##
============================================
+ Coverage     80.80%   85.80%   +4.99%     
============================================
  Files           281      298      +17     
  Lines         12659    15875    +3216     
============================================
+ Hits          10229    13621    +3392     
+ Misses         2430     2254     -176     

Impacted Files	Coverage Δ
contentcuration/contentcuration/decorators.py	56.60% <50.00%> (-37.60%)	⬇️
...ntentcuration/contentcuration/db/models/manager.py	91.20% <90.98%> (-8.80%)	⬇️
contentcuration/contentcuration/forms.py	82.35% <94.04%> (+33.89%)	⬆️
contentcuration/contentcuration/api.py	92.06% <100.00%> (+1.43%)	⬆️
...tentcuration/contentcuration/context_processors.py	100.00% <100.00%> (ø)
...ontentcuration/contentcuration/db/advisory_lock.py	100.00% <100.00%> (ø)
...tcuration/contentcuration/db/models/expressions.py	93.33% <100.00%> (-6.67%)	⬇️
...entcuration/contentcuration/db/models/functions.py	100.00% <100.00%> (ø)
...ation/contentcuration/tests/test_rest_framework.py	36.93% <0.00%> (-63.07%)	⬇️
contentcuration/contentcuration/utils/format.py	21.05% <0.00%> (-49.54%)	⬇️
... and 181 more

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