Add property for GraphiQL UI path

This commit adds a new `spring.graphql.graphiql.path` configuration
property for setting the path to the GraphiQL UI page.

This change requires updates in the static HTML page to link the
JavaScript fetcher back to the actual GraphQL API endpoint.

Closes gh-70

README.md

@@ -125,6 +125,9 @@ spring.graphql.schema.location=classpath:graphql/schema.graphqls
 # endpoint path is concatenated with the main path, so "/graphql/schema" by default
 spring.graphql.schema.printer.enabled=false
 spring.graphql.schema.printer.path=/schema
+# GraphiQL UI configuration
+spring.graphql.graphiql.enabled=true
+spring.graphql.graphiql.path=/graphiql
 # whether micrometer metrics should be collected for graphql queries
 management.metrics.graphql.autotime.enabled=true
 ````