Add locale selector to log in page

[richard-cox]

The goal of this ticket is to add the Locale Selector to the login page of dashboard.

Currently it is only available in the slide-in menu once you are logged in (bottom-right of the slide-in).

I suggest adding it to the bottom-left of the login page - you should be able to leverage the control straight from the slide-in menu - see: TopLevelMenue.vue around line 362. You will need to extract that into a new component that can be used both in the TopLevelMenu and the login screen (shell/pages/auth/login.vue).

On the login screen, we don't know who the user is, so we can't retrieve their locale preference from the backend.

I suggest that we cache the locale in browser local storage

On the login screen:

• If there is no cached locale, we default to English, otherwise we use the locale that has been cached
• When the user changes the locale on the login screen, we update the cache and store in the app that the locale was changed
• Once the user logs in, we will have their locale preference from the backend
• if the locale was changed on the login screen, we update the user preference with the new locale - this will then be stored by the backend
• If the locale was not changed, we update the cached locale in the browser with the value from the user preference if this is different to what is currently cached