small bugfixes (#2655)

* improve report visibility
* fix timesheet duration, which breaks validation
* added contributing section in README
* do not allow to change project for existing activities - fixes #2576
* bump theme version to fix FOSUserBundle dependency
* use entity color instead of inherited one - fixes #2200

src/Entity/Timesheet.php

@@ -411,6 +411,11 @@ class Timesheet implements EntityWithMetaFields, ExportItemInterface
      */
     public function getDuration(): ?int
     {
+        // only auto calculate if manually set duration is null - the result is important for eg. validations
+        if ($this->duration === null && $this->begin !== null && $this->end !== null) {
+            return $this->end->getTimestamp() - $this->begin->getTimestamp();
+        }
+
         return $this->duration;
     }