Fail when inserting change log item in released version

[lenny0702]

Take https://github.com/zattoo/web/pull/3276 as an example, when the new release is cut on master, it would be nice for changelog action to remind the author that the changelog item shouldn't be inserted into an already released version.

I did a bit of research and found it seems possible to get line numbers of changes of the PR using this way. So we can check if PR is changing the content of an already released version. And I am very happy to contribute since I feel I quite easily forget to update the changelog when a new release is cut.

@dobladov What do you think?