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/properties.gradle

@@ -16,4 +16,5 @@ ext {
     junitJupiterVersion = '5.1.0'
     mockkVersion = '1.7.15'
     assertkVersion = '0.9'
+    detektVersion = '1.0.0.RC8'
 }