[display] render exclusive write arrows

[uforic]

Description

I noticed that exclusive write arrows weren't being rendered when using the render_dot_tree function.

In the code that adds edges, we check reads and writes, but not exclusive writes.

Adding exclusive writes fixed the issue.

Open questions

• What color should exclusive writes be? Read is green, write is blue. Any other visual differences we'd prefer?
• Per the contribution guide, I was hoping to add labels to this PR, but don't seem to have permissions. Anything I'm doing wrong here?

[stonier]

Set the color to deepskyblue and incidentally, found and fixed a bug with rendering off-the-grid blackboard clients.

Changes added in #383.

Thanks!

Also removed the reference to labels in CONTRIBUTING.md. Did not realise those without write access could not edit the labels. Thanks for the catch.