fix #485: avoid crash due to duplicate screen id

[zqianem]

Closes #485

What are the key elements of this solution?

This fix takes option (a) from the issue and maps the F8 key to nothing once the history screen is open.

Why did you design your solution this way? Did you assess any alternatives? Are there tradeoffs?

An alternative is to bind F8 to the cancel action or nothing in the history screen class, but that makes it awkward as the F8 binding would be then defined in two places.

Does this PR require a change to Harlequin's docs?

• [x] No.
• [ ] Yes, and I have opened a PR at tconbeer/harlequin-web.
• [ ] Yes; I haven't opened a PR, but the gist of the change is: ...

Did you add or update tests for this change?

• [x] Yes.
• [ ] No, I believe tests aren't necessary.
• [ ] No, I need help with testing this change.

Please complete the following checklist:

• [x] I have added an entry to CHANGELOG.md, under the [Unreleased] section heading. That entry references the issue closed by this PR.
• [x] I acknowledge Harlequin's MIT license. I do not own my contribution.