Better handling of location in locale source

Follow up to https://github.com/backdrop/backdrop-issues/issues/704:

Seems to be that locale.module remembers the path at which the translated string was first seen, then provides a path to that location? Perhaps that's so that you can try and find if a string is used on that page. However... not very useful for something like site_name which is literally on every page. If we can find a way to suppress this behavior when strings are translated through config, that would be preferable. Most config strings will probably be shown in lots of places (e.g. field labels and descriptions, view names, block titles, etc)

For reference, in D8 there's now a separate table for locale location and it better represents config strings:

• https://www.drupal.org/project/drupal/issues/1777070
• https://api.drupal.org/api/drupal/core%21modules%21locale%21src%21String...