feat: add check for changelog entries

[r0b1n]

This adds check of what is changed in the changelog files, prevents merging broken changelogs.
This will fail if:

• A changelog entry added into existing version (they always have to go to Unreleased section)
• This will fail if two sections with the same name exist in the Unreleased section (For example ### Fixed added two times because of rebase issues)
• This is skipped for uicontent user, so this won't affect automated PRs.