Identifiers for PeriodO

Before merging a patch that adds new period collections or period definitions (via "add" or "replace" operations), any skolem URIs in the patch are replaced with relative URIs. Relative URIs for collections consist of 4 random alphanumeric chars (leaving out vowels, 0, and 1) plus a check character so that they can be easily validated. Relative URIs for definitions consist of the URI for the collection to which they belong, plus `/`, followed by 3 random alphanumeric chars and a check character.