Link to online map theme legends

[moving-bits]

After having removed the builtin map theme legends, we should link to the external sources. Strings with links are already part of c:geo, but they not visible on the UI yet. Implementing them as part of the "Theme options" dialog may be a feasible solution.

This issue should probably wait until conceptual discussion about map menu and its implementation are finished (see #15847 / #16127).