android,ios: add store screenshots

While Fastlane recommends automagic screenshots, we cannot really do that since
we require a specific setup for a video call and so on.

.gitignore

@@ -61,14 +61,8 @@ buck-out/
 
 # fastlane
 #
-# It is recommended to not store the screenshots in the git repo. Instead, use fastlane to re-generate the
-# screenshots whenever they are needed.
-# For more information about the recommended setup visit:
-# https://docs.fastlane.tools/best-practices/source-control/
-
 */fastlane/report.xml
 */fastlane/Preview.html
-*/fastlane/screenshots
 
 # Build artifacts
 *.jsbundle