Add a java language standard

[MatMoore]

This is a first stab at putting together some common guidance for java teams at the MOJ.

It's heavily based on the GDS way guidance.

We've made the following changes:

• Promote Google Style Guide as the default format instead of Intellij defaults. I don't think Intellij is a good fit for MOJ because you can't easily apply the same conventions to other IDEs/static analysis tools. We can potentially expand on this to suggest other alternatives, but I think this is a decent starting point and this was the main recommendation I got when I first asked around about coding standards on slack.

• Take out the section about dependency injection and web frameworks - this is better suited to the tech menu and MOJ use the spring ecosystem whereas GDS don't

• Take out the section about imports, this should be covered by the google style guide

• Take out the section about testing, as we have another page for that

• Slightly rewrite the section about the JDK. I think this is quite confusing and I want to explicitly recommend going with the latest JDK version unless there's a good reason not to.

[MatMoore]

https://ncrcoe.gitbooks.io/java-for-small-teams/specifics.html also has some good advice that's a bit more high level than the google style guide, but I didn't think it worth going into that much detail in our guidance, otherwise it will get quite long.

[MatMoore]

Have moved a lot of that code style into the linting rules page, which cuts down on a lot of duplication.

I spent about half an hour trying to get checkstyle to use built in configuration files, but I couldn't figure out how to make it work with the gradle plugin. According to this post it can be done but I gave up and just documented the method we've got working instead.