Fix "Convert to work item" button UI bugs

What does this MR do and why?

This MR fixes some Convert to work item button UI bugs, some of which were raised in !85936 (comment 945953119).

It fixes the Convert to work item button:

1. Incorrectly showing for child tasks when hovering over parent tasks
2. Sometimes not showing to the right of the task text
3. Having poor vertical alignment
4. Showing when the user does not have permissions

Screenshots or screen recordings

before	after
Screen_Recording_2022-05-25_at_5.01.34_pm	Screen_Recording_2022-05-25_at_5.02.27_pm

Before

How to set up and validate locally

1. In rails console enable the FF

   Feature.enable(:work_items)

2. Visit an issue page such as http://127.0.0.1:3000/h5bp/html5-boilerplate/-/issues/1
3. Update the description with task list items and test the MR

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.