Upgrade JID class to RFC 7622 / Allow German letters in resource parts

German "Umlauts" like ü, ö, ä, ... are currently disallowed by the JID class, leading to the following error, which prevents users to login to Openfire with resources containing german umlauts, e.g. "Büro" (== "office").

Resource parts in JIDs follow the rule:

The resourcepart of a JID is an instance of the OpaqueString profile
of the PRECIS FreeformClass, which is specified in RFC7613

The FreeFormClass is specified here and allows "LetterDigits" which include the Unicode category "Ll - Lowercase_Letter".

"ü", "ä", "ö", etc. are in this Unicode category and therefore should be allowed in resource parts.

The JID class should be upgraded to RFC 7622 because it uses the old RFC 6122 and the obsolete Stringprep spec, which has been superseded by PRECIS.

(This issue can probably be moved to Tinder project, but I have no rights to do so).