Default title for govuk_tabs

[frankieroberto]

The govuk_tabs helper throws an exception if the title argument is omitted. The equivalent Nunjucks component govukTabs defaults the title to "Contents" and doesn’t even include it in the examples.

I’m happy for us to require arguments where it makes sense to (Nunjucks doesn't support required arguments at all with the way that arguments are passed in as a object) – but maybe here we could default to "Contents" too? Not sure how often you'd use another title?

See also #506