Gradle and maven snippets can be externalised

* Maven templates go in _includes/maven_{dependency,repository}.xml
* Gradle templates go in _includes/gradle_{dependency,repository}.gradle

Example maven_dependency.xml

    <dependency>
        <groupId>{@= groupId @}</groupId>
        <artifactId>{@= artifactId @}</artifactId>
        <version>{@= version @}</version>
    </dependency>

[#55283992]