Update PULL_REQUEST_TEMPLATE (#2139)

Author: renefloor

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,4 +1,8 @@
 # Submit a pull request
+<!--Internal tickets have to be added by Stream devs-->
+Linear: FLU-
+<!--Optional to add github issue which is solved by this PR-->
+Github Issue: #
 
 ## CLA
 
@@ -7,3 +11,15 @@
 - [ ] Code changes are tested (add some information if not applicable)
 
 ## Description of the pull request
+<!-- 
+Describe how these code changes fix the issue or how the feature works.
+Also try to give instructions how this can be tested.
+-->
+
+## Screenshots / Videos
+
+<!-- Consider to add screenshots and/or videos to show the effect of the change -->
+
+| Before | After |
+| --- | --- |
+| img | img |