Change URL from /category/list to /categories (and the same for countries)

[php-coder]

It turned out that URLs have been chosen wrongly. They don't conform to REST but also create issues like #509 and #510

Let's change them (but keep redirect for backward compatibility).

[php-coder]

You need to modify the following classes:

• CategoryController
• CountryController
• Url

Here is an example of making permanent redirect.

[php-coder]

@n583wxdh0j I'm unassigning the issue since there is no activity. Let me know if you're working on it or have a plan to finish it.