Make it possible to navigate the menu with the keyboard. Fix #202

[chris-lawton]

Description

Currently the pattern library uses the menu item heading tags to listen for events which open and close their child menus. This is not accessible for users who navigate via keyboard only.

This MR resolves that by changing the menu item elements to buttons. In turn it also resolves https://github.com/torchbox/django-pattern-library/issues/202.

It also:

• Moves the padding on .sidebar to a new element .sidebar__inner as when the sidebar is collapsed the padding meant the elements within the sidebar were still visible.
• Updates a Tox config option: whitelist_externals is replaced by allowlist_externals - https://tox.wiki/en/latest/changelog.html#deprecations-and-removals-4-0-0rc4

Fixes #202

Checklist

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [x] I have added an appropriate CHANGELOG entry