Fix link to Creative Commons
There is problem with license text, it doesn't affect Esperanto, but causes problems with other languages. Currently the website provides link to https://creativecommons.org/licenses/by/4.0/deed.XX where XX is Pepper&Carrot-website-code of the language, which is not always consistent with ISO-codes. I think, that it should be extracted to translation (file "_catalog.pot"), so translators will provide the most relevant language of the license (for example Silesian will provide deed.pl, Mexican Spanish deed.es, etc.)
Raised in !37 (merged)