Make it possible to post related events

[gchpaco]

The current version of dogshell interprets --related_event_id as a string, not an integer, which means the backend refuses to work with it. The fix is simple; to tell argparse that it's an int.

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What does this PR do?

Fix #785 by adding typing to related event IDs in argparse.

Description of the Change

To ensure that the JSON API is sent an integer and not a string, we add type=int to the relevant argparse declaration.

Alternate Designs

You could do it at the call site, or otherwise parse it, but argparse makes this relatively easy.

Possible Drawbacks

If you were in the habit of posting random garbage to related event ID and expecting it to fail, it will now fail slightly earlier.

Review checklist (to be filled by reviewers)

• Feature or bug fix MUST have appropriate tests (unit, integration, etc...)
• PR title must be written as a CHANGELOG entry (see why)
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