Use HTTP protocol properly

[php-coder]

• We have to change URLs to describe resources, other parameters should go to a query part:
  • /account/activate/key/{key} -> /account/activate?key={key}
  • /series/add/category/{slug} -> /series/add?category={slug}
  • /series/add/country/{slug} -> /series/add?country={slug}
• "Search by catalog" form should use GET method instead of POST because it doesn't change state
• We should return 403 error code instead of 401 for unauthenticated requests because 401 must have WWW-Authenticate header

Inspired by article (in Russian): https://habrahabr.ru/company/yandex/blog/265569/