Fix encoding issue happening when providing URLs containing UTF-8 characters

This commit fixes an exception raised by ERB in `i18n_viz/settings.js.coffee.erb` when an `external_tool_url` is passed with UTF-8 characters.

For instance:

I18nViz.external_tool_url = "https://webtranslateit.com/en/projects/406/locales/en..fr/strings?utf8=✓&s="

See related issue: #2