Update MR template for spotlights

Adds some minor improvements to the Spotlights template:

• reminder to reference the issue
• a little explanation for the review app
• milestone added
• set @marco.de.lucia as primary reviewer and mention @christian.meessen

changed milestone to %Spotlights

requested review from @jandt-desy

assigned to @jandt-desy

added Progress::4_Can review label

mentioned in commit 5346aebe

merged