Remove apollo-plugin Gradle plugin

Replaces the old Gradle plugin with plain Gradle buildscripts.  Also cleans-up
the current Gradle extensions and re-adds detekt, along with JUnit 5 support to
all modules.

gradle/kotlin.gradle

@@ -1,10 +1 @@
-apply plugin: 'org.jmailen.kotlinter'
-
-kotlinter {
-    ignoreFailures = true
-    indentSize = 4
-    continuationIndentSize = 4
-    reporters = ['checkstyle', 'plain']
-}
-
 kotlin { experimental { coroutines 'enable' } }