Replace upload service with purpose plugin

Summary:
Purpose is the modern way of doing this. This will make the upload action show up in Dolphin's (and other apps) Share menu.

Test Plan: Trigger share from Dolphin

Reviewers: Choqok, volt4ire, scarpino

Reviewed By: Choqok, scarpino

Maniphest Tasks: T12256

Differential Revision: https://phabricator.kde.org/D25722