Reposition revert and cherry-pick buttons on MR widget

This issue is part of https://gitlab.com/gitlab-org/gitlab-ce/issues/33095

Design/Proposal

from:

to:

• Revert and cherry-pick buttons will become smaller as they will be inline buttons by default
• They will shift position to be behind the "merged" message.
• They can remain their current position for mobile (if easily done)

cc: @victorwu (making the https://gitlab.com/gitlab-org/gitlab-ce/issues/33095 more readable) @psimyn