Add the [Enter] shortcut to the modal window for entering duplicates issue
## Issue
On the issue management page, you can mark an issue as a duplicate. In this case, a modal window for entering a URL will appear. Pressing <kbd>Enter</kbd> in it does nothing.
| 1 | 2 |
| ------ | ------ |
|  |  |
The window should accept the entered URL when <kbd>Enter</kbd> is pressed in the input field.
## Implementation plan
Changes will mostly be needed in `app/javascript/pages/manage-issue-page/CloseDuplicateModal.vue` and probably `app/javascript/pages/manage-issue-page/CloseDuplicateModal.spec.js`
1. Remove the default footer from the modal (using the `no-footer` prop like we do in `app/javascript/pages/user-page/AddPointsModal.vue`)
2. Add a BForm wrapper around the form (handling the `@submit` event)
3. Add a BButton submit button
4. Update tests
Alternatively, we COULD consider adding some kind of listener for enter key, but I don't think that's the right way to go about it.
BONUS: We could also research whether anyone has already implemented a solution for bootstrap-vue-next and/or is there an open issue in their issue tracker?
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/cc @gitlab-org/developer-relations/contributor-success
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