Merge branch 'feature/changelog-entries-generator' into 'master'

Add changelog entries generation script

## What does this MR do?

Adds a script that helps to automatically prepare entries for `CHANGELOG.md` 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=3e1ec38c ./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

See merge request !322