Use translation contexts

Some English strings are ambiguous for translation, and should be translated differently in different contexts. For example, "Log in" is used both as the submit button on the form and the menu link to the form (in addition to "Login" which refers to the user name).

Unfortunately jinja2 doesn't support contexts yet, but it's in progress: https://github.com/pallets/jinja/pull/1126. Let's add those when it's released.