ulo-storage-endpoint: add /explore and /explore/contributor pages

...dpoint/src/main/java/info/mathhub/uloapi/html/Base64.java

...endpoint/src/main/java/info/mathhub/uloapi/html/Main.java

...dpoint/src/main/java/info/mathhub/uloapi/html/Routes.java

...int/src/main/java/info/mathhub/uloapi/query/Literals.java

...dpoint/src/main/java/info/mathhub/uloapi/query/Query.java

...int/src/main/resources/spark/template/freemarker/base.flt

...nt/src/main/resources/spark/template/freemarker/error.flt

.../src/main/resources/spark/template/freemarker/explore.flt

...sources/spark/template/freemarker/explore_contributor.flt

...ulo-storage-endpoint/src/main/resources/uloapi.properties