Donate and language switch in top bar nav

[ajparsons]

This PR:

• Adds a link to the mySociety donate page in the top bar.
• Moved the language selector to the top right (https://github.com/mysociety/theyworkforyou/issues/1727)

Doesn't entirely solve #1727 because of it doesn't change the hamburger approach.

Donate approach is a bit hacky - didn't want to add it to the metadata file because it's not a page. Is weird to have an external link in the normal navigation - but it's how we've got things set up at the moment.