Add a license statement in the footer (#637)

* Add a license param in the config used in footer

Also adds initial EN and NL i18n statements referring to the license.

* Add Michiel to CONTRIBUTOR

* Add example text for i18n of `licensed_under`

* Add safeHTML filter to license value in the footer

Also updates the example config in the exampleSite config.toml to link
to Creative Commons.

* Fix separator dot in footer within License ifdef

i18n/bn.toml

@@ -30,6 +30,9 @@ other = "দুঃখিত, কাঙ্ক্ষিত পাতাটির
 [head_back]
 other = "আপনি <a href=\"{{ . }}\">নীড়পাতায়</a> ফিরে যেতে পারেন"
 
+[licensed_under]
+other = "Licensed under"
+
 [powered_by]
 other = "চালনায়"