Implement script to streamline various checks performed by the integration manager before a new release

Typically when preparing a new release, the integration manager must:

• Check all the issues on develop branch are correctly reported in Changelog.md
• Check the issues for which there are commits are mentioned in Changelog.md (possibly not as closed ones - but they should appear as "ongoing issue" in this case)
• Check there are no remaining todo #id where id is the id of an issue to be closed.

Not all of these checks can be completely automated (especially for Changelog itself, where I sort issues by sub-library which are not mentioned so far in Gitlab) but the point is to automatize what can be to ease my task.

python-gitlab will be used to query Gitlab.