Restore missing Toolbar for KSP 0.25

Toolbar-1.7.7 for KSP 0.25 no longer seems to be at the original site.
This PR flips to using a mirror (thank goodness for free licenses) so
0.25 users can still install Toolbar and its mods.

Attn @blizzy78, as we'd really rather download from your site directly.

Relates to #169, which I'll be processing in a minuteto provide
1.7.8/0.90 support.