Publish Gradle Plugins to gradlePluginPortal

if `grails-gradle-plugin` is published to gradlePluginPortal,

`buildSrc/build.gradle`
`settings.gradle`
and a `buildscript { }`

are no longer needed for Grails Apps.

Also remove need for
```gradle
apply plugin:"org.grails.grails-web"
apply plugin:"org.grails.grails-gsp"
```

grails-gradle-plugin currently consists of
```gradle
org.grails.grails-core
org.grails.grails-doc
org.grails.grails-gsp
org.grails.grails-plugin
org.grails.grails-profile
org.grails.grails-web extends org.grails.grails-plugin
org.grails.internal.grails-plugin-publish
org.grails.internal.grails-profile-publish
```

`org.grails.grails-web` should automatically be applied if `org.grails:grails-web-url-mappings` is present
`org.grails.grails-gsp` should automatically be applied if `org.grails:grails-gsp` is present