Update default MR template with quick actions and SAFE/get approval handbook URLs

Why is this change being made?

While recording the practical handbook edits workflows with the new Web IDE and static file editor, I figured that the quick actions at the bottom of the MR default template are always done manually. Today I created a new MR and saw that the /assign quick action had been added :-)

This MR adds more common quick actions to the default MR template, such as the default handbook label (I would guess that 95% of updates are handbook updates). I have also added the SAFE checks, and when to get approval as tips with URLs. The mention of maintained by is helpful already, but the workflow is more complicated than just assigning a codeowner or manager.

Author Checklist

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added.
• Assign reviewers for this MR to the correct Directly Responsible Individual/s (DRI)
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the Maintained by section on the page being edited
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies
• If the changes affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

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