Clean Theme Navigation Fixes

[StephDriver]

From screenreader testing (#4194), looking at the narrow screen navigation the following issues were identified:

• Sample 3, item 5. menu items with dropdowns don't distinguish themselves from those which are links to other pages.
• Sample 3, item 6. articles item (read out as "articles, link") opens a dropdown the first item of which is articles (read out as "articles, link). This is highly confusing!
• Sample 3, item 7. likewise the second item under 'submissions' is also called 'submissions'
• Sample 3, item 8. and again, 'About' has the first subitem as 'about'
• Sample 3, item 9. Login and Register are a new list of two items, but part of the menu navigation landmark... confusing.

Dev notes
This a11y issue stems from the fact that top level nav items can have a path/link associated with them, while also having subnav. In order to provide the dropdown menu on click events, while also allowing the link to be navigable, the submenu renders a duplicate of the top level nav item.

Instead we have agreed the following actions:

• Make has_sub_nav and link mutually exclusive
• To add a data migration that will update any nav items that meet both conditions and wipe the link attribute. A sub nav item will be created using the same name and link as its parent.