test: Add tests for EmailOnPushService

Add support for email on push integration.

fix: #612 Update path for delete commit discussion note

Fixed ClassCastException when getting label name

feat: add support for GitLab merge request reviewers

Update AccessLevel.java

On gitlab.com I got 
```
org.gitlab4j.api.models.AccessLevel forValue
WARNING: [5] is not a valid GitLab access level.
```
so I've added that enum here

Added option to preserve underscores in the JacksonJsonEnumHelper to fix TargetType.MERGE_REQUEST value. (#615)

Remove UserListSerializer and UserListDeserializer from approvedBy field in ApprovalRule (fixes #589) (#590)

* Added path and name to fork project

Co-authored-by: Thomas van Putten <thomas@bit-academy.nl>

Fixes #595