Add PR creator as issue assignee when a PR is opened

[koppor]

Triggered by #12745 -> #12747

Currently, issues are unassigned when a PR is closed. If a contributor submits a new PR, the issue is marked as "in-progress", but the assignee is not updated.

With this PR the assignees of an issue are "enrichted" by the PR submitter.

I am aware that an issue have then multiple persons assigned. I think, this helps rather then it hurts.

Mandatory checks

• I own the copyright of the code submitted and I licence it under the MIT license
• Change in CHANGELOG.md described in a way that is understandable for the average user (if change is visible to the user)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (if change is visible to the user)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

[github-actions]

The build for this PR is no longer available. Please visit https://builds.jabref.org/main/ for the latest build.

[subhramit]

Not very familiar with programming github actions but lets see how this turns out