Add public domain license to footer

[bitmask]

Website should show that the content is in the public domain, look into copying how DEAL licenses user contributions. When people contribute, they should know their content is going into the public domain.

[samuk]

It might be necessary to do this on a per-map basis. For example https://transition.resilienceweb.org.uk/ needs to be ODbL licenced as it queries that API