Gradle wrapper should be executable

When downloading exercises in the kotlin track (and maybe java as well?), which contain gradle wrapper files (gradlew and gradlew.bat) - those files are non-executable and must be chmoded before mentees can run them.

This was discusses in https://github.com/exercism/cli/issues/276 before.

To me, this doesn't make much sense. Either include an executable gradle wrapper or avoid including it altogether, but giving the mentees a wrapper that they need to chmod before being able to run it means we need to instruct them on how to do so and makes solving solutions more complicated than it needs to be.

Start by locating gradlew and gradlew.bat in the Kotlin exercises and checking their executable permissions in downloaded exercises. Review the earlier discussion in exercism/cli#276 for context. Done means downloaded exercises either provide an executable Gradle wrapper or omit it, without requiring mentees to chmod it.

Written by the indexing model from the issue text.