Allow users to delete group invitations

[mbramson]

Currently group invitations are only removed when they are accepted. There is no way to deny a membership request or an invite, all a user can do is ignore it forever.

Implement the :delete action for both the UsersGroupMembershipRequestController and the GroupInvitationController.

Allow the owners of groups to delete GroupMembershipRequests using the :delete action on the GroupInvitationController which should be located on the :index action's template.

Allow users to delete their own GroupMembershipRequests using the :delete action on the UsersGroupMembershipRequestController which should be located on the :index action's template.

• Implement controller actions
  • :delete action on GroupInvitationController
    • A GroupMembershipRequest associated with a Group should only be deletable by the group's owner.
  • :delete action on UsersGroupMembershipRequestController
    • A GroupMembershipRequest associated with a User should only be deletable by that user.
• Add delete links to the UI
  • Delete links on GroupInvitationController index.html.eex template.
  • Delete links on UsersGroupMembershipRequestController index.html.eex template