Since targets didn't have a title but only a long description, every
form allowing to select targets was pretty much unusable: we either
displayed just the code or the whole description.
Now, with a concise title, it's easier to find and select the desired
target.
The titles have been copied from The Global Goals page [1].
Note we're using the `short_title` I18n key for the `title` method and
the `long_title` I18n key for the `long_title` method. We can't use
`title` as I18n key instead of `short_title` because it would affect
existing translations.
[1] https://www.globalgoals.org/
c025fef50b