Add GitLab support for MR merged events

[SantiMA10]

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

Implementation plan

• new /gitlab endpoint feat: new /gitlab endpoint #153
• new MergeRequestMerged reaction for GitLab feat: new 'MergeRequestOpened' reaction #156
• add MergeRequestMerged reaction to /gitlab endpoint feat: 'MergeRequestOpened' and 'MergeRequestMerged' #160
  new "Ping" reaction for GitLab