Fix work items sidebar glitch incase of parent widget

What does this MR do and why?

While reviewing the UX of parent widget in the MR !133426 (merged), it was observed the work item sidebar was shifting a bit while interacting with widgets like Assignee. The widget needs some space when interacted. This MR adds width to the sidebar.

Screenshots or screen recordings

Before	After
Screen Recording 2023-10-17 at 3.52.56 PM.mov	Screen Recording 2023-10-17 at 3.53.31 PM.mov

How to set up and validate locally

Prerequisites:

1. Enable the OKRs feature as mentioned in the OKRs docs (e.g. ::Feature.enabled?(:okrs_mvc, Project.find(<PROJECT_ID>)) and the namespace must have Ultimate plan).

Steps to verify:

1. Login with any user
2. Go to any Project > Issues > List
3. Create an new Objective under New issue split button
4. Go to Objective detail page
5. Interact with the Assignee widget to check if the sidebar is not shifted.

In addition to above flow, it also affects the Key results and Tasks feature.

For Key results/Tasks:

1. Add a Key result/Task as mention Add a key result docs/Add a Task docs under an Objective/Issue
2. Open the Key result/Task
3. Interact with the Assignee widget to check if the sidebar is not shifted.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #421611 (closed)