revise the tab upload dialog to direct people at the kde store

Summary:
previously the dialog would ask users to mail John Tapsell who I think
doesn't maintain ksysguard anymore, plus that was weirdly not scaling.
instead we can simply publish it through the kde store (D8734).
provide all pertinent information to do this.

also
- switch to xi18n and use markup tags instead of manual rich text
- use KNS to retrieve the category name for the store so as to not
  hardcode it
- assert that the KNS config has only one category to ensure this doesn't
  go randomly messed up in the future should multiple categories get used

Test Plan:
- create a new tab
- file -> upload
- dialog pops up
- dialog has relevant information to actually publish the file

Reviewers: #plasma, jriddell, apol

Reviewed By: apol

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D8735