Add integration tests for supporting backward compatible URLs

[php-coder]

Tech debt for #514:

• /account/activate/key/{key} -> /account/activate?key={key} (6351bb8)
• /series/add/category/{slug} -> /series/add?category={slug} (e579d53)
• /series/add/country/{slug} -> /series/add?country={slug} (e579d53)
• /series/add?category={slug}&country={slug} (e579d53)

[php-coder]

As it isn't fixed for about 3 years, I'm going to remove support for these URLs (see #1120), so this issue won't be fixed.