feat: Add ability to pick preferred locale for a user

[danjohnson95]

This PR allows selecting the preferred locale for a user.

It also adds a middleware that sets the application locale to the user's preferred locale, or the system default (as specified by the browser in the Accept-Language header) if one isn't set.

I've specified the supported locales in the config file, and this will be used for:

• The options which a user can pick from
• Which locale to use when the browser sends multiple locale preferences

Changing the locale changes the language the user sees and the way in which dates and numbers are formatted in.

The application uses the fallback locale if a string doesn't have a translation defined for the given locale.

Going forward, if any new translation strings are added, the locale code should be added to the supported_locales config file.

This change only affects the Dashboard.

[jbrooksuk]

Thanks Dan, this is super cool!