gs-license-tile: Add a ‘Learn More’ link to the proprietary tile

This links to a wiki page where we can explain the differences between
FOSS and proprietary software in more detail. This means that we can
explain more of the nuances and risks without being limited by the word
limit in the license tile widget.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

Helps: #1421
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