Fix empty string in attribute description in ldap

- if description is empty string, work with it like with null
- use in resource and group
Merge pull request #231 from stavamichal/ldap_fix_empty_string_in_description

Fix empty string in attribute description in ldap
GUI: fixed behavior on member's status change

- Cell displaying membership status will now refresh itself,
  so we don't have to refresh whole table.

- Unified MembershipStatus widget implementation and fixed
  events chain, that was broken on member's detail tab.

- Remember checkbox state and display loading state in table
  when tab is refreshed using refresh button in top-right corner.

- Pass checkbox state to "list all" callback, when user check it
  first and then load vo members.
Merge pull request #232 from zlamalp/members

GUI: fixed behavior on member's status change