Handle notifications when merging MRs

[Moussto]

Hey !

I figured I'd add two snackbar notifications (success and error) when trying to merge Mrs, there's no real error handling per se, but its a start (helps a bit with #16 )

If you want to try it out I suggest you mock a failed api call in merge_request.js inside the merge action with

merge({ dispatch }, mr) {
    return Promise.reject(new Error('fake error'))
    // const gl = gitlab.get();
    //
    // return gl.client.merge(mr).then(() => {
    //   dispatch("removeMergeRequest", mr);
    //   // TODO: Stop watchers for this MR
    // });
  }

Don't hesitate to suggest changes or offer feedback :)