[Duplicate] Feature/citations

[LinseCed]

Important

Duplicate of #114

Related issue

#59 (See comment below issue)

Short summary

Citations are now persisted in the backend, meaning that even after reloading or changing to another chat, they remain visible in the frontend messages.

Checks

• I verified the code makes sense intuitively
• The PR changes affect only this issue, no unrelated/unwanted code changes to other modules/code segments
• CI runs (./gradlew clean build, keycloack)
• New business logic is unit tested, including WebMvcTest for api controllers
• The new functionality is tested manually

Additional notes

Conflicts may (most likely will) appear in regard to PR #113, which introduces changes in the same files as this PR. Should be fixable without efforts, though.

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Note: this is a fresh branch (feature/citations-dev-backup) rather than a reuse of the original feature/citations head, since that branch had dev merged into it (to resolve real conflicts with #113) and reusing it here would have pulled in #113/#115/#116 as well. This branch is a clean cherry-pick of just #114's own changes on top of dev-backup.