[kicker/kickoff] Use discover icon for manage application action

Summary: While not 100% technically correct since another app might be associated with appstream URLs it's still much more expressive than the current icon

Test Plan: {F7302971}

Reviewers: #plasma, hein, ngraham

Reviewed By: ngraham

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D23623