Replace changelog generator script

What does this MR do?

Changes the way how CHANGELOG.md entries generation is done.

Example entries generated by a `make generate_changelog CHANGELOG_RELEASE=v12.9.0-rc1` call:

diff --git a/CHANGELOG.md b/CHANGELOG.md
index cb289fccb..827fe9ce8 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,53 @@
+## v12.9.0-rc1 (2020-03-02)
+
+### New features
+
+- Add trigering of GitLab Runner UBI images pipeline !1869
+- Update docs for image variable in custom executor !1866
+- Add execution stage name in job trace !1847
+- Provide rpm/deb package for arm64 and aarch64 !1826
+- Expose CI_JOB_IMAGE env var on build environment !1813
+- Overwite kubernetes resource limits and requests for build container on job level !874 (Nicholas Colbert @45cali)
+
+### Technical dept reduction
+
+- Rename build makefile target to build_all !1873
+- Prevent building mips and s390x architectures by default !1872
+- Make pipelines running also for X-Y-stable branches !1871
+- Add double quotes around bash arguments in ci/release_docker_images !1865
+- Fix release docker warning !1864
+- Fix typo in dev docs !1862
+- Simplify sha256 checksum file creation !1859
+- Improve fpm detection !1858
+- Replace which command usage !1857
+- Convert indentation to spaces for package script !1856
+- Update synced path for Windows 10 machine !1854
+- Use chocolatey to install software in Vagrant boxes !1853
+- Remove redundant type declaration !1852
+- Bump to go 1.13.8 !1849
+- Add debug logs when setting up pod !1844
+- Set DinD image explicitly to 19.03.5 !1840
+- Resolve data race in TestCommand_Run !1839 (Konrad Borowski @KonradBorowski)
+- Document merge request title requirements and reviewing guidelines !1836
+- Use $(MAKE) instead of make !1825
+
+### Documentation changes
+
+- Prevent full disk image copies in libvirt custom executor driver example !1878 (Tom Dohrmann @Freax13)
+- Interactive Web terminal does not work on Windows !1877 (Ben Bodenmiller @bbodenmiller)
+- List which executors are at risk of having Runner token & other project code stolen !1876 (Ben Bodenmiller @bbodenmiller)
+- Remove bash from codeblock tags !1846
+- Improve wording in docs/development/README.md !1837
+- Add documentation on building from sources !1835
+- docs-update-security-document !1833 (masOOd @masood.kamyab)
+- Update the monitoring document !1831 (masOOd @masood.kamyab)
+- Change flag to docker-services !1830
+- Create network per build in docker executor !1569 (Steve Exley @steve.exley)
+
+### Other changes
+
+- Bump Minio to latest version !1881 (Tom Elliff @tomelliff)
+
 ## v12.8.0 (2020-02-22)
 
 - Define most jobs as 'pipelines for merge requests' !1747

Why was this MR needed?

1. Reduce code duplication. Currently we use a changelog generation script that is implemented separately in each of our projects. The same was done with S3 index page generator, which we recently extracted to a separate, dedicated tool. This is a follow-up step for this.

2. Make our changelog cleaner, by grouping entries by the type of the changes (like features, fixes, security fixes, documentation etc.). As for the changelog entries themselves, all information are taken from the Merge Requests, using labels to scope the entries and decide whether to add the author name or not (following https://docs.gitlab.com/ee/development/changelog.html#overview, we will be using this to give attribution to community contributors).

Are there points in the code the reviewer needs to double check?

This MR will need to be synced with gitlab-org/charts/gitlab-runner!209 (merged), which will change the https://gitlab.com/gitlab-org/ci-cd/runner-release-helper's template that contains step for the changelog entries generation.

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Added tests for this feature/bug
• In case of conflicts with master - branch was rebased

What are the relevant issue numbers?