Group changelog entries

1. Make CHANGELOG.md more human readable.
2. Refactor markdown_generator.rb and entry.rb.

Example of markdown

## 10.1.7 (2017-11-09)

### Fixed (18 changes, 1 of them is from community)
- Auto Devops kubernetes default namespace is now correctly built out of gitlab project group-name. !14642 (Mircea Danila Dumitrescu)
- Forbid the usage of `Redis#keys`. !14889
- Make the circuitbreaker more robust by adding higher thresholds, and multiple access attempts. !14933
- Only cache last push event for existing projects when pushing to a fork. !14989
- Fix bug preventing secondary emails from being confirmed. !15010
- Fix broken wiki pages that link to a wiki file. !15019
- Don't rename paths that were freed up when upgrading. !15029
- Fix bitbucket login. !15051
- Update gitaly in Gitlab 10.1 to 0.43.1 for temp file cleanup. !15055
- Use the correct visibility attribute for projects in system hooks. !15065
- Fix editing issue description in mobile view.
- Adds callback functions for initial request in clusters page.
- Normalize LDAP DN when looking up identity.
- Fix missing Import/Export issue assignees.
- Allow boards as top level route.
- Fix widget of locked merge requests not being presented.
- Fix deletion of container registry or images returning an error.
- Fix the writing of invalid environment refs.

### Changed (2 changes)
- Store circuitbreaker settings in the database instead of config. !14842
- Update default disabled merge request widget message to reflect a general failure. !14960

### Performance (1 change)
- Stop merge requests with thousands of commits from timing out. !15063

Closes https://gitlab.com/gitlab-org/release-tools/issues/110