feat(materials): add 404 folder not found page

[cysjonathan]

resolves issue #7437 by showing a 404 screen when visiting non-existent folders instead of an empty materials table
supersedes PR #7612

• opens up new /materials/folders route which returns course's root folder data
• when accessing a non-existent folder, 404 screen will appear (similar to non-existent file)
• other minor changes --- add rspec for material folder show, drop trailing slash in folder links in FE

[purfectliterature]

Very neat PR, except for,

• the controller calling other controller action, and
• anti-patterned getFolderTitle definition.

The rest are nitpicks and some questions.

With the new /materials/folders route, technically we can already replace the path for Workbin on the sidebar. This should reduce about 1-2 SQL calls since there's no need to resolve current_course.root_folder in main_sidebar_items in Course::MaterialsComponent. Just need to ensure that activePath (hence the active highlight on the sidebar) still works even after this change. But this isn't urgent, and the behaviour this PR brings is already good as it is; it can be another good first issue for others.

[bivanalhar]

@cysjonathan the code now has been made much cleaner, just one more comment to be addressed, to make the redirection to Root Folder much clearer and easier to understand

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