Mobile optimisations MR-widget

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

Design/Proposal

This issue will probably need additional UX due to many different states and functions being displayed within the MR widget. However the following design will be used as a starting point into improving the MR widget for smaller viewports, especially mobile.

Let's see if we can implement the following, and look as to how things will react.

Note: this issue builds upon the changed of other issues in https://gitlab.com/gitlab-org/gitlab-ce/issues/33095

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