Implement Discord slash commands handler in API client

[spalmurray]

Provide a method in the Discord API client that takes a list of slash commands and ensures the slash commands registered with Discord matches.

We'll need to

• ~fetch the currently registered commands~ (not necessary)
• ~delete any commands that are missing from the input list~ (not necessary)
• do a bulk global command overwrite with the input list

Finally, once this is implemented, make sure we call it in the setup method of our Discord IntegrationProvider. This way the list of registered commands always matches what we have defined in our repo.

[github-actions[bot]]

This issue has gone three weeks without activity. In another week, I will close it.

But! If you comment or otherwise update it, I will reset the clock, and if you label it Status: Backlog or Status: In Progress, I will leave it alone ... forever!

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