Fix collaborators modal (add + delete editors)

[rbennettcw]

Fixes issue where the thread collaborators modal did not properly add editors. Also ensures that it can properly delete editors.

Link to Issue

Closes: #3008

Description of Changes

• Updates the request body shape in addEditor and deleteEditor endpoints
• Updates the collaborators modal UI to send a properly shaped request body
• Updates the collaborators modal UI to update the UI after collaborators have been updated

Test Plan

• Create a thread
• Add a collaborator, confirm that the UI is updated to show the new collaborator
• Remove the collaborator, confirm that the UI is updated to show no collaborators

Deployment Plan

N/A

Other Considerations

I noticed that when updating partial state, there are instances of mutating the state directly like this:

thread.foo = 'bar';
setThread(thread);

It's generally not good practice to mutate state directly, but since Thread is a class and not a plain object, we can't use the spread operator. As a result, we'll need to continue applying the mutation pattern. Just an FYI though I'm sure we all know this already.

[masvelio]

this is not good. We are now in the transition between handling global state from mithrill world and handling local react state. This mixes two approaches but we should strive to follow react approach. In this case this would be the approach which hopefully works:

const newThread = {...thread, collaborators: newEditors};
setThread(newThread)

[masvelio]

We should definitely move away from mutating => this is a legacy and we cant write new code in the legacy approach. Other than that, looks great!