Incident timeline separator ends early

Summary

The separator lines between the rows of events are ending early:

Probably introduced in !105240 (merged) due to HTML structure changes.

Steps to reproduce

1. Navigate to Monitor > Incidents page
2. Create new or open an existing incident
3. Navigate to Timeline tab of the incident
4. Click "Add new timeline event" button at the bottom
5. Add at least two events per one date

Example Project

What is the current bug behavior?

Timeline event separator goes only under the note of the event.

What is the expected correct behavior?

Timeline event separator should go under the note and the action button of the event.

Relevant logs and/or screenshots

Output of checks

Results of GitLab environment info

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(For installations with omnibus-gitlab package run and paste the output of: \`sudo gitlab-rake gitlab:env:info\`) (For installations from source run and paste the output of: \`sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production\`)

Results of GitLab application Check

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(For installations with omnibus-gitlab package run and paste the output of: `sudo gitlab-rake gitlab:check SANITIZE=true`)

(For installations from source run and paste the output of: `sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true`)

(we will only investigate if the tests are passing)

Implementation Guide

You might need to move a separator to a container to avoid any width hacks. Mind that in !108129 (merged) a gap for the grid is set. But the separator should only be under the 2nd & 3rd grid item.