Implement method getGroupSubjects for ExtSourceXML

- use group attribute groupMembersQuery for searchString
Merge pull request #274 from stavamichal/addMethodGetGroupSubjectsToXMLExtSource

Implement method getGroupSubjects for ExtSourceXML
GUI: fixed user detail from perun admin view

- Fixed passing user to change password dialog trigerred from user detail page (perun admin section)
- Make content of "Certificates, Logins, Passwords" scrollable in case there is a lot of certificates etc.
Merge pull request #275 from zlamalp/pass

GUI: fixed user detail from perun admin view