fix(form/variables/standalone): persist submitted variable

related to CAM-3141
chore(welcome): remove cycle screenshot
fix(engine): add function mapper if expression manager already set

related to #CAM-3143
fix(distro): start-camunda works with spaces in path

related to #CAM-2624
test(engine): test function mappers for custom expression manager

- relevant for modules like engine-spring and engine-cdi that exchange the
  expression manager; for these, the built-in function mappers should get
  registered

related to CAM-3143