Be consistent with using plural/singular for item paths under the "Structure" menu

[klonos]

This may be part of the #2118 issue, which will most likely need to happen in 2.x, and its only goal is cleanup/consistency (so relatively low priority I guess) - we could drop it altogether if we don't really care about that, but I thought that we should at the very least have an issue to discuss and decide. That way, we'll have something to refer people to in the future if this is brought up 😉

Here's what that menu looks like currently:

• Content types (admin/structure/types)
• Custom blocks (admin/structure/block)
• File types (admin/structure/file-types)
• Layouts (admin/structure/layouts)
• Menus (admin/structure/menu)
• Taxonomy (admin/structure/taxonomy)
• Views (admin/structure/views)

There's 2 parts of this: the human-readable menu item label, and the "machine name" of the path:

• When it comes to the labels, the only "pariah" is "Taxonomy" (using singular instead of plural). Perhaps this should be better decided in the older [UX] Rename "taxonomy" to "Categorization" in the user interface #203 (where there's already previous discussions about renaming the menu item).
• When it comes to the paths though, there's quite a few issues:
  • "Content types" uses just types for its path. If we wanted to be consistent, we'd rename that to content-types (similar to file-types).
  • "Custom blocks" uses block (singular). It should be blocks or custom-blocks instead. I would argue that that would need to be renamed to "Reusable custom blocks" or just "Reusable blocks", but that's most likely a matter to be discussed in [UX] Come up with better terms for "custom blocks" and "reusable blocks" #5922.
  • "Menus" uses menu (singular). It should be menus instead.
  • "Taxonomy" will need to be discussed and decided in [UX] Rename "taxonomy" to "Categorization" in the user interface #203, and this issue here should be taken into account when it comes to consistency (although that is a developer-focused thing - UX and user-facing text/terminology should be prioritized).

Internal redirects should be added from the old paths, so to not break things.

To keep things moving and avoid debates/bike-shading over one specific item blocking progress on the rest, we may need to make it a meta-issue, with separate sub-issues for each task, or keep it a single issue and simply file multiple PRs (one for each task). Although low priority, certain of the tasks here may be simple to decide and work on, so these low-hanging fruits (like changing menu to menus and block to blocks for example) could be baby steps towards gradual improvement/consistency - we don't have to decide and action this whole thing in a single go.