We were writing the same code twice, with the only difference being the
text "Sign up" in the sign_up action, and "Sign in" in the sign_in
action.
Note we're renaming the `omniauth.info_*` I18n keys so we don't need to
add new exceptions to the `ignore_unused` list, and so it's consistent
with all the other keys under the `omniauth` key.
Sometimes we're interpolating a link inside a translation, and marking
the whole translations as HTML safe.
However, some translations added by admins to the database or through
crowdin are not entirely under our control.
Although AFAIK crowdin checks for potential cross-site scripting
attacks, it's a good practice to sanitize parts of a string potentially
out of our control before marking the string as HTML safe.
Using the block syntax to generate the label with a <span> tag inside
isn't necessary after upgrading foundation_rails_helpers. Before the
upgrade, we couldn't do so because the <span> tag was escaped.
