Reduce hit target of more memory loader

[colin-grant-work]

What it does

Fixes #134

The previous code didn't set the header wrapping div to be flex, and it didn't justify content itself, meaning that the 'more memory loader' took up the full width.

How to test

1. Confirm that the hit target of the more memory loader is limited to the text itself.

Alternatively, the more memory renderer could wrap itself so that it still took up the whole width, but only the text was the hit target. If this seems preferable, it can be implemented.

Review checklist

• [x] as an author, I have thoroughly tested my changes and carefully followed the contribution guidelines

Reminder for reviewers

• as a reviewer, I agree to behave in accordance with the code of conduct

[jreineckearm]

Thanks a lot, @colin-grant-work , for picking this up! @arekzaluski , could you please help with reviewing this one?

[colin-grant-work]

@arekzaluski, good catches. I've made a new issue (#144) for those.