Carpentries: fix "More" drop down moving into hamburger side menu when screen width is reduced

[steve-crouch]

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One to communicate with the Carpentries:

The "More" drop down to get to learner extras moves into the hamburger side menu at the bottom when the screen width is reduced beyond a certain limit. @douglowe and @bielsnohr felt this wasn't great UX and could possibly be improved.

1. The "More" dropdown on a screen of "normal" desktop width. Notice the theme and learner view toggles just above.

2. On a narrower screen, everything gets collapsed into the "Menu" toggle (which is fine).

3. Problem: upon opening the menu, only the theme and learner view options are displayed at the top of this menu, when one would expect to see the Glossary, Key Topics, etc also listed here.

4. Instead, the extras are put at the bottom of this menu panel.