Create UI for sending mails to group's members

User interface for sending mails to group's members should be based on parent User Story (POULPE-443):

• The new context menu item "Send mail" should be created in Poulpe->User Groups->group's context menu(right click on any group)
• After choosing "Send mail" item new dialog box should appear with title "Send message to #GROUP_NAME members". This dialog box should contain next elements:
  a) "Message's topic" field - to define message's topic
  b) "Message's body" field - to define message's body
  c) "Send" button - to send letter to group members
  d) "Cancel" button - to cancel letter sending and redirect to Poulpe -> User Groups
• While the functionality is not implemented, the message "The functionality is not implemented in scope of http://jira.jtalks.org/browse/POULPE-443" should appear after "Send" button click.
• If selected group is empty the context menu item "Send mail" should be disabled.