Update map_feature_is_writable to check tree permissions

[jwalgran]

This addresses an edge case where a role has permission to add trees but not add plots.

I made use of a helper function to check tree creatability so I renamed it from map_feature_is_creatable to model_is_creatable.

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Testing

• Create a new instance.
• Enable Can add tree on the Public role.
• Invite a secondary user to the Public role.
• As the admin user, create an empty plot.
• Log in as the secondary user and go to the empty plot detail page.
  • The Edit button should be enabled.
  • Clicking Edit should show the Add tree button, but none of the plot fields should be editable.
  • Saving the new tree succeeds.

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Connects to #2856

[coveralls]

Coverage increased (+0.02%) to 83.87% when pulling fa5200d627268be42113930014c7601d9e500bbc on jwalgran:feature/update-plot-writable-check into 52a89f63dcaae2c0235c9376f2a1d170b757eaa5 on OpenTreeMap:develop.