feature: basic editable responsive menu

[jbmoelker]

Changes

• Adds an AppMenu to the default layout header. The menu items are editable via the CMS (new models included in migration). The menu has basic responsive behaviour with a full and compact mode. See AppMenu/README.md for details.
• Adds icons for the open and close menu buttons.
• Refactors inline logic to small components so they could be reused in multiple places (AppLogo, IconButton, SearchLink and UnstyledList).
• Moves components that should only be used in layouts to src/layouts/, just like the new AppMenu.
• Adds / updates 7 small READMEs.

Only AppMenu is really new. All the other changes are assets (icons), refactors and documentation. So the number of files changed may be significant. But the actual changes are not that big.

Note: other types of menu items (like external links) and nested menu items can be added in a separate PR so this one doesn't become any bigger.

Associated issue

Part of #196.

How to test

1. Open preview link
2. Verify the header contains the new app menu with items from the CMS
3. Verify the menu items work
4. Verify that when the menu items no longer fit, the menu items are hidden and a menu button appears.
5. Verify the menu button opens the menu in a dialog and you can operate it (focus management, tab and esc work).
6. Change the menu in the CMS
7. Verify the new menu items are rendered (requires local dev server)

Checklist

• I have performed a self-review of my own code
• I have made sure that my PR is easy to review (not too big, includes comments) (only AppMenu is really new, other files are moved around and many short READMEs are added)
• I have made updated relevant documentation files (in project README, docs/, etc)
• I have added a decision log entry if the change affects the architecture or changes a significant technology
• I have notified a reviewer

[cloudflare-workers-and-pages]

Deploying head-start with    Cloudflare Pages

Latest commit:	518df0d
Status:	✅  Deploy successful!
Preview URL:	https://41daa761.head-start.pages.dev
Branch Preview URL:	https://feat-basic-menu.head-start.pages.dev

View logs

[jurgenbelien]

I feel that the close behavior of the menu dialog is a bit to eager in closing itself.