Move the about page from Discover to Kirigami

Summary:
Includes the AboutPage we're using in Discover for other Kirigami
projects to use.
It also includes UrlButton and LinkButton components that I had in Discover for
longtime and can also be useful here.

Test Plan: Used the AboutPage from Discover

Reviewers: Kirigami, mart, broulik

Reviewed By: Kirigami, mart

Subscribers: leinir, nicolasfella, plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D17216