Render different headings for Work Item when isModal is false

What does this MR do and why?

Adjust the headings used for work items to be semantically correct for situations where a WI is rendered as its own page.

• use H1 for title not H2
• use H2 for Activity not H3
• use text styles that match legacy issuables
• update tests

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Check out this branch
2. visit an issue that has tasks
3. Open a task in a modal (click on the task name)
   1. confirm that the title is in an H2
   2. confirm that the Activity header is an H3
4. close the modal and command-click the task title to open it in a new tab
   1. confirm that the title is in an H1
   2. confirm that the Activity header is an H2; it will look different from the small H3 used in a modal. It should match the Activity header used on a legacy issuable.

MR acceptance checklist

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