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/
This is similar to what we do with investments, which belong to a heading
but also belong to a budget. In our case, the reason is we've been asked
to add local targets which belong to a goal but are not related to any
existing target.
Even though we're not implementing that case right now, we're adding the
relation so we don't have to add data migrations in the future.
