Add changelog entries generation script

Merged Tomasz Maczukin requested to merge feature/changelog-entries-generator into master

What does this MR do?

Adds a script that helps to automatically prepare entries for file.

Script assumes that a GITLAB_PRIVATE_TOKEN is set and it contains users API access token.

If nothing else is set it will find all merge requests from the latest stable tag (a tag with a vX.Y.Z pattern). However there may be a situation (like for v1.6.0 release) that the previous stable tag is created not in the current branch history. In such case the STARTING_POINT may be used to set a SHA which should be used, eg. STARTING_POINT=3e1ec38 ./scripts/prepare-changelog-entries.rb

If there is a MR that was next reverted, we can use the EXCLUDE_MR_IDS variable, eg. EXCLUDE_MR_IDS=123,456,777 ./scripts/prepare-changelog-entries.rb

Why was this MR needed?

While preparing a release process description we've decided that it would be good to automatically generate CHANGELOG entries basing on merge requests titles.

What are the relevant issue numbers?

!176 (merged)