in order to determine whether someone has been “deprived of his liberty” within the meaning of Article 5 of the ECHR, “the starting point must be his concrete situation, and account must be taken of a whole range of criteria such as the type, duration, effects and manner of implementation of the measure in question. The difference between deprivation of and restriction upon liberty is merely one of degree or intensity, and not one of nature or substance. [...] Above all, such confinement must not deprive the asylum-seeker of the right to gain effective access to the procedure for determining refugee status” (para. 43)