Merge: Popcorn: more validators

Add two more validators:
* URLField that ensures a field contains a valid URL (you want to redefine that regexp).
* UniqField stubs for uniq emails, logins, ids validation.

How to validate uniq field:

~~~nit
class UniqEmailField
	super UniqField

	var users: UsersRepository

	redef fun check_unicity(v, field, val) do
		var user = users.find_by_email(val)
		if user != null then
			v.validation.add_error(field, "Email `{val}` already used")
			return false
		end
		return true
	end
end
~~~

Pull-Request: #2435

diff --cc lib/popcorn/pop_validation.nit
Simple merge