Bump newsroom-core from a8eb8a9 to 7840513 in /client

Bumps [newsroom-core](https://github.com/superdesk/newsroom-core) from `a8eb8a9` to `7840513`.
- [Release notes](https://github.com/superdesk/newsroom-core/releases)
- [Commits](superdesk/newsroom-core@a8eb8a9...7840513)

---
updated-dependencies:
- dependency-name: newsroom-core
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>