Add validation and make mandatory fields visible

Some fields and mandatory while others are optional. Would be good to show the mandatory ones using xwiki validation practices and validate fields.

See https://www.xwiki.org/xwiki/bin/view/Documentation/DevGuide/Tutorials/Creating%20a%20form%20with%20validation%20and%20tooltips

Note: I'm not an expert in this field, best would be to check this with other xwiki committers on how to do this.