Welsh translation

[struan]

Update WTT to enable translations and translate into Welsh.

Main tasks are:

• remove unused templates - there seem to be a few legacy templates so we might as well remove them
• work out how we are going to switch - cy.wtt a la twfy?
• set language for PHP based on above
• add language column to messages table
• save which language the site was set to when creating a message
• respect message language in queue operations (will need a default for existing messages)
• make sure admin is always in English, including errors from Welsh messages
• make sure sending messages from admin respects message language
• wrap errors and messages in queue in translation function
• update queue email template fetching to use language
• wrap front end content in translation functions
• set up email templates for translation (won't be able to use gettext due to the templating code)
• update tests for language
• generate po files for translation
• scripts to manage translation files
• get translations

For the email templates I'm proposing that we have an emails/$lang/ type structure which will mean some duplication but avoids rewriting a lot of the email formatting/handling code.

I'm also not sure if some of the more verbose php pages might be better handled by loading language specific templates rather than using gettext. Largely because I can see translating them piecemeal might be annoying. I'm thinking specifically of the various help pages.

It's also unclear if we need to translate absolutely everything into Welsh (e.g content about MSPs is probably not likely to come up), but I suspect we should just do everything regardless.

[ajparsons]

This is great! On the help pages - we also want to overhaul these anyway - so probably does make sense to treat as a separate problem (we might want an approach that's more customisable by where you are to some of the help pages anyway).