Support translations

[tunetheweb]

Makes progress on #671

This makes the following changes:

• If a requested template is a supported language, but the template does not exist it 302 redirects back to English version. This allows partial release of translated templates/chapters.
• For each template, at rendering time we test each supported language and check if the translated template exists, and only list language if it has been translated. This means the language drop down only shows languages were a translation exists
• Makes the Language drop down show always - even if only English exists
• Adds border for Language dropdown always and not just on hover (I found it didn't look like a button without the border)
• Adds a link to Github to ask for help with Translations
• Removes top "Start Exploring" link form home page as discussed in Optimize website for wide viewports #574 as doesn't fit when Language Drop down is shown
• Adds a note to ToC and Prev / Next links to note if chapter has not been translated (see screenshot)
• Removes Japanese placeholder templates so more obvious these need translated

Screenshots:

Home Page Widget:

Home Page Widget with Drop down open:

Chapter page drop down with only English:

Table of Contents notes which chapters have been translated into that language:

Note on French chapter for previous chapter not existing:

With this I think we're good to launch the French translations. Will open a separate PR for that.