refactor(webapp): change invite code button and menu to copy invite link

Created by: mahula

🍰 Pullrequest

Change invite code button.

Issues

• fixes #5978 (closed)

Todo

• add a tooltip like on the group or map button: Invite your friends
• in the opened menu:
  • change the text to: Copy Invite Linkand leave the code away
  • center the button
• change the short description above to: Send this link per e-mail or in social media to invite your friends: