Add GitLab support for MR opened events

[SantiMA10]

docs: https://docs.gitlab.com/ee/user/project/integrations/webhooks.html

Implementation plan

• new MergeRequestOpened reaction for GitLab feat: new 'MergeRequestOpened' reaction #156
• add MergeRequestOpened reaction to /gitlab endpoint feat: 'MergeRequestOpened' and 'MergeRequestMerged' #160