Update elisa icon

Summary:
Since D12992 seems stuck, update the current icon
a little bit and add a blueish gradient to the
icon. Also remove the ugly text for the 48px icon,
since it was not readable anyway at small sizes.

Reviewers: VDG, ngraham, mgallien, trickyricky26

Reviewed By: VDG, ngraham, mgallien, trickyricky26

Subscribers: trickyricky26, GB_2, kde-frameworks-devel

Tags: Frameworks, Elisa

Differential Revision: https://phabricator.kde.org/D24598