investigation: issue priority should be treated as string or integer consistently.

This could be a bug, or it could be a test needing update. Reading `api/hooks/issues-hook/update-work-item-priority/index.js`, issue priority is treated numerically, and can be compared with higher-priority issues are `>` than lower-priority.

```js
const isNewPriorityGreat = issue && priority > issue.priority;
```

However, the test code for this passes in priority strings like "P1" and "P2". Strings like these compare the other way around, `"P2" > "P1"`, even though P1 should be higher-priority.

From `api/hooks/issues-hook/update-work-item-priority/__test__/updateWorkItemPriority.spec.js`,

```js
Issues.getByRelatedUrl.mockReturnValue(
Promise.resolve({
id: '1234abcd',
priority: 'P1'
})
);
```

Resolve this by looking at production logs or GUS api responses, and update either the test code or the implementation code. Consider adding type checking.